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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
Comprehensive reference on the use of single-element semiconductor materials, such as carbon, silicon, and others, in modern electronic devices
Single Element Semiconductors discusses the preparation, properties, and applications of single-element semiconductor materials in modern electronic devices. Special attention is paid to the nanostructures which show great potential in the fields of energy, electronics, and sensing due to their unique physicochemical properties.
Written by a highly qualified team of researchers, Single Element Semiconductors includes information on:
- Properties and fabrication of carbon nanomaterials, germanium, borophene, stanene, arsenene, and antimonene
- Synthesis, self-assembly, and in-plane epitaxy of horizontal silicon nanowires
- Blue, black, and violet phosphorus and the controlled synthesis of tellurium nanomaterials
- Most suitable applications for each element semiconductor, including in photodetectors, solar cells, batteries, biomedical sensors, and photocatalysis
Single Element Semiconductors is an ideal reference for researchers and engineers seeking to advance the research and application development of single-element semiconductor materials.
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Seitenzahl: 542
Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Chapter I: Carbon
1.1 Introduction
1.2 Fabrication of Carbon Nanomaterials
1.3 Properties and Applications of Carbon Nanomaterials
1.4 Conclusion
References
Chapter II: Silicon
2.1 Introduction
2.2 Synthesis of Si NWs
2.3 Horizontal Si NWs: Self-assembly or In-plane Epitaxy
2.4 Applications of Si NWs
References
Chapter III: Germanium
3.1 Introduction
3.2 Synthesis of Germanium Nanomaterials
3.3 Properties and Applications of Germanium Nanomaterials
3.4 Conclusion
References
Chapter IV: Phosphorus
4.1 Introduction
4.2 Synthesis of BP
4.3 Synthesis of BP Nanosheets
4.4 Properties of BP
4.5 Applications of BP
4.6 Blue Phosphorus
4.7 Violet Phosphorus
4.8 Conclusion
References
Chapter V: Tellurium
5.1 Introduction
5.2 Controlled Synthesis of Tellurium Nanomaterials
5.3 Properties and Applications of Tellurium Nanostructures
5.4 Conclusion
References
Chapter VI: Selenium
6.1 Introduction
6.2 Synthesis of Selenium Nanomaterials
6.3 Properties and Applications of Selenium Nanostructures
6.4 Conclusion
References
Chapter VII: Borophene, Stanene, Arsenene, and Antimonene
7.1 Introduction
7.2 Borophene
7.3 Stanene
7.4 Arsenene and Antimonene
7.5 Conclusion
References
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 The relationship between graphene and other carbon nanostructures.
Figure 1.2 (a) OM image of FLG produced using original micromechanical cleavage ...
Figure 1.3 (a) Schematic of a pressure–temperature phase diagram showing the su...
Figure 1.4 (a) Schematic of a typical dual-electrode ECE setup.
Figure 1.5 (a) Schematics of catalyst-assisted exfoliation of HCDG. (b) XRD pat...
Figure 1.6 (a) Schematic of a CCS furnace with two SiC chips inside a graphite ...
Figure 1.7 (a) Schematic of CO
2
-assisted BLG growth through the ....
Figure 1.8 (a) Chiral vector of a SWCNT in the unfolded...
Figure 1.9 (a) Schematic of CNT synthesis using AD. (b) High-resolution TEM ima...
Figure 1.10 (a) Schematic of CNT synthesis using LA. (b) TEM image of SWCNTs wit...
Figure 1.11 (a) Schematic of ultralong CNT synthesis using SIDS. (b) CFD simulat...
Figure 1.12 GY containing different numbers of inserted acetylene bonds.
Figure 1.13 (a) Experimental SAED pattern and (b) schematic stacking mode of ABC...
Figure 1.14 (a) Schematic of GDY synthesis by decarboxylation coupling reaction....
Figure 1.15 (a, b) Schematics of (a) CVD system for GDY growth on Ag and (b) the ...
Figure 1.16 (a) 3D schematic of the post-strain cycling process of GSET. (b) SEM...
Figure 1.17 (a) Hall coefficient of FLG versus at .
Figure 1.18 (a) Schematic of Au-Al
2
O
3
-MLG-Ag/Au-Al
2
O
3
stacked ...
Figure 1.19 (a) Schematic of split-gate BLG THz PD. (b) Comparison of current ...
Figure 1.20 (a) Schematic of a dual-gate WSe
2
/ABCA-4LG device. (b, c) ...
Figure 1.21 (a) Four-probe resistance of two devices, M1 and M2, with ...
Figure 1.22 (a) Photograph of single-chirality SWCNT solutions. (b) SWCNTs with ...
Figure 1.23 (a) False-color SEM image of three top-gated CNT-FETs in series with ...
Figure 1.24 (a) False-colored SEM image of aligned CNT PD with a channel length ...
Figure 1.25 (a) Transfer characteristics of ML-GDY FET at . Inset: Output characteristics ...
Chapter 2
Figure 2.1 Schematic diagram of VLS and VSS growth. (a) Schematic representatio...
Figure 2.2 Patterning of seeds. (a) Thin film thermal annealing. (b) Gold seeds...
Figure 2.3 Transportation of precursors. (a) Schematic of low-temperature CVD. ...
Figure 2.4 (a–f) Schematic phase diagrams of different metal-Si systems. (a) Au...
Figure 2.5 Left: theoretically, there exist nine variations of diameters. Right...
Figure 2.6 Analysis of contact angle and surface tension. (a–e) Scanning electr...
Figure 2.7 (a) Schematic diagram of the thermal evaporation growth device. A ...
Figure 2.8 OAG based on pretreated silicon surface. (a) Schematic diagram of VS ...
Figure 2.9 Schematic representation of the core–shell ...
Figure 2.10 Si NW solar cells. (a) Three representative phenomena of the ...
Figure 2.11 (a) Schematic of the volt-ampere characteristic curve of a solar cel...
Figure 2.12 Passivation of Si NW solar cells. (a) Schematic of the multicore-she...
Figure 2.13 Basic understanding of silicon-based lithium batteries and electrode...
Figure 2.14 (a) Schematic of Si c-a core–shell NWs lithiation grown directly on ...
Figure 2.15 Fabrication and on-chip integration of Si NW TEGs. (a) Porous Si NW ...
Figure 2.16 Electrical properties of Si NWs. (a) Calculated results: the bandgap...
Figure 2.17 Summary of transistor types that can be realized with Si NWs. Compar...
Figure 2.18 Development history and emerging trends of MOSFETs with different tr...
Figure 2.19 Si NW planar FET process. (a) Etching and self-limiting oxidation.
Figure 2.20 Vertical GAA SNWT with PtSi/Si contacts that enable dense arrays. ...
Figure 2.21 (a–d) SEM side views of the different process steps after completion...
Figure 2.22 Fabrication of 3D-stacked Si NWs: etching approach. (a) Schematic an...
Figure 2.23 Fabrication of 3D-stacked Si NWs: growth methods. Guided Si NW growt...
Figure 2.24 Directional merging dynamics related to droplet tilt angle. (a–c) Sc...
Chapter 3
Figure 3.1 (a) TEM image of Ge NWs and a selected-area electron-diffraction ...
Figure 3.2 (a) TEM micrograph of a long, 18-nm diameter Ge single-crystal ...
Figure 3.3 (a) SEM image of Ge NWs. (b) SEM image of Ge NWs grown from 10 nm Au...
Figure 3.4 SEM images of the Ge nanostructures deposited at a substrate tempera...
Figure 3.5 (a) SEM image of Ge NWs on a Fe substrate. (b) HRTEM image of an epi...
Figure 3.6 (a) SEM image showing high-density, uniform Ge NWs grown from a plan...
Figure 3.7 (a) SEM image of the products.
Figure 3.8 The typical field-emission scanning electron microscopy images of Ge...
Figure 3.9 PL spectra under 488 nm laser excitation and excitation spectra at t...
Figure 3.10 Comparison of the extinction spectrum.
Figure 3.11 First-order Raman scattering from Ge at 300 K. The dashed curve repr...
Figure 3.12 (a) Raman spectra of the Ge nanostructures deposited at different su...
Figure 3.13 (a) Specific capacity versus cycle number for nano/microstructure Ge...
Figure 3.14 (a) First voltage profiles of 0D hollow and 3D porous NP assemblies ...
Figure 3.15 (a) First, second, third, and twenty-sixth charge–discharge curves f...
Figure 3.16 I-V measurements of Ge/HfO
2
/TiN capacitor structures. The...
Figure 3.17 (a, b) Characteristics of high-performance Ge/Si NWFET.
Figure 3.18 (a) Drain current–drain voltage output characteristics of the juncti...
Chapter 4
Figure 4.1 Schematic diagram of the experimental setup for the production of BP...
Figure 4.2 Schematic of BP preparation by mineralization method using SnI
4
...
Figure 4.3 Schematic diagram of the BP-stripping process in NMP.
Figure 4.4 (a) BP synthesis process flow. (b) Schematic diagram of the device f...
Figure 4.5 Schematic diagram of solvothermal synthesis of BP.
Figure 4.6 (a) Side view of crystalline BP. (b) Top view of monolayer BP.
Figure 4.7 Variation of bandgap with thickness of oligolayer BP.
Figure 4.8 Hall mobilities measured at a constant hole doping concentration of ...
Figure 4.9 (a) Schematic diagram of a photoelectric FET.
Figure 4.10 (a) Schematic diagram of the BP/ReS
2
heterostructure on ...
Figure 4.11 (a) Three-dimensional illustration of the device configuration, feat...
Figure 4.12 Structure of the mid-infrared PD based on tunable BPs.
Figure 4.13 (a) Schematic of BP-ZnO heterojunction device fabricated on glass su...
Figure 4.14 (a) SEM cross-sectional image of planar n-i-p chalcogenide solar cel...
Figure 4.15 Schematic illustration of the synthesis of BPQDs and their potential...
Figure 4.16 (a) Photoluminescence spectra of eda-BP, pda-BP, and bda-BP showing ...
Figure 4.17 Atomic model of blue phosphorus. a and b are the jagged edges of ....
Figure 4.18 (a) Asymmetric unit of violet phosphorus. (b) Layered structure of v...
Chapter 5
Figure 5.1 (a) TEM images of Te nanoseeds. (b) HRTEM image of a single seed lemon.
Figure 5.2 (a) TEM images and (b) HRTEM image of the obtained Te sample.
Figure 5.3 (a) TEM images of tellurium nanotubes at three different stages of g...
Figure 5.4 (a–c) TEM images of tellurium nanosheets. (d) HRTEM images.
Figure 5.5 (a) Schematic of vapor deposition of tellurene on SiO
2
/Si ...
Figure 5.6 (a) HRTEM images of tellurene. (b) 3D illustration of the structure ...
Figure 5.7 (a) Chiral system with helical structures.
Figure 5.8 2D tellurene FET device performance. (a) Transfer curve of a typical ...
Figure 5.9 (a) Photoresponse spectrum of Te NW device under the laser ...
Figure 5.10 (a) Band structure of Te. (b) The curve of the current to the pressu...
Figure 5.11 (a) Optical image of a typical Pt contact touching Te NW. (b) ...
Figure 5.12 Resistance responses of tellurene sensors at room temperature. The t...
Figure 5.13 (a) TEM image of AgSeTe NWs. (b) HRTEM image of AgSeTe NWs. Inset: A...
Figure 5.14 (a) Schemes for illustrating various cell configurations of TABs. Ty...
Chapter 6
Figure 6.1 (a) 2D AFM image of selenium NPs formed by the reaction of ...
Figure 6.2 (a) TEM image of a representative selenium NP with its size ∼85 nm. ...
Figure 6.3 TEM image of several Se NWs assembled parallel to each other.
Figure 6.4 (a) TEM image of the Se NWs. The diameters of the Se NWs vary in the...
Figure 6.5 (a) TEM image of selenium nanotubes prepared in ethylene glycol by B...
Figure 6.6 (a) SEM image of selenium rods prepared at 130°C.
Figure 6.7 (a) TEM image of Se nanobelts obtained in C
18
EO
20
micellar solution.
Figure 6.8 (a) TEM image of the as-prepared 2D nonlayered Se nanosheets obtaine...
Figure 6.9 (a) UV-vis absorption spectra of Se nanobelts obtained in C
18
EO
20
mi...
Figure 6.10 (a) Representative polarized (VV) Stokes-side Raman spectrum of ...
Figure 6.11 (a) PL spectra of selenium NPs synthesized using leaf extract.
Figure 6.12 (a) Photocurrent density of Se PD under various power intensities in...
Figure 6.13 (a) FET properties of a single Se NW: graph of versus for gate voltage...
Figure 6.14 (a) J–V curves of Se solar cells.
Figure 6.15 (a) Effect of Se-NRs on the viability of Hep-G2 and MCF-7 human ...
Chapter 7
Figure 7.1 (a) SEM image of the borophene.
Figure 7.2 (a, b) Raman spectra of the borophene.
Figure 7.3 (a, b) A comparison between the electrochemical performance of the s...
Figure 7.4 (a) STM image of the stanene.
Figure 7.5 (a) Electronic structure of stanene.
Figure 7.6 (a) Isosurface plot of the electron charge density difference for ...
Figure 7.7 (a) TEM images of arsenene.
Figure 7.8 (a) Raman spectra of arsenene and (b) photoluminescence spectra of a...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Single Element Semiconductors
Properties and Devices
Professor Yi Shi
Professor Shancheng Yan
Authors
Professor Yi Shi
Nanjing University
No. 163 Xianlin Da Dao
Qixia District
Nanjing
CH, 210023
Professor Shancheng Yan
Nanjing University of Posts and Telecommunications
No. 9 Wenyuan Road
Qixia District
Nanjing
CH, 210023
Cover Design: Wiley
Cover Image: © asharkyu/Shutterstock
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Preface
In the rapidly developing field of nanotechnology, single-element semiconductor materials have become a research hotspot due to their unique physical and chemical properties and wide application prospects. The purpose of this book is to discuss in depth the preparation, properties, and applications of single-element semiconductor materials in modern electronic devices. Special attention is paid to the nanostructures formed by elements such as carbon, silicon, germanium, phosphorus, and tellurium, which show great potential in the fields of energy, electronics, and sensing due to their unique physicochemical properties. The book will provide the latest research progress on the representative structures of graphene, silicon nanowires, black phosphorus, and other elements, including their preparation methods, electronic and optical properties, as well as the application prospects in energy storage, optoelectronics, biomedicine, and other fields. The purpose of this book is to provide a comprehensive reference resource for researchers and engineers to advance the research and application development of single-element semiconductor materials. In this book, we not only review the historical development of single-element semiconductor materials but also focus on their synthesis strategies and application prospects. We believe that the unique properties of these materials will revolutionize future electronic devices. With the deepening of research, the potential of these materials will be further tapped and contribute to scientific and technological progress.
Chapter ICarbon
Yi Shi
1.1 Introduction
Carbon (C) is a nonmetallic element of period 2, group 14 (group IVA) of the periodic table, and the fourth most abundant element in the universe [1]. There are four valence electrons (2s22p2) outside the carbon atom, meaning that all three types (sp, sp2, sp3) of hybridization between s- and p-orbitals could occur in carbon atoms during the formation of chemical bonds. Due to the bonding characteristics, millions of organic compounds in which carbon atoms usually bond with other nonmetallic atoms (e.g. hydrogen, oxygen, nitrogen, etc.) or the C atom itself (forming carbon chain, branch, or ring structures) have been discovered in nature or synthesized artificially.
For elementary substances of carbon (not regarded as organic molecules but inorganic ones), different hybridization modes between carbon atoms lead to the diversity of structure and properties of its allotropes. Consider two typical types of carbon bulk materials: diamond is a three-dimensional (3D) network composed of sp3-hybridized carbon, known as the hardest natural material, while graphite is a 3D structure stacked by a sp2-hybridized carbon monolayer with a low degree of hardness but good lubricity. Carbon nanomaterials include diverse low-dimensional allotropes of carbon, such as graphene [2], graphene nanoribbons (GNRs) [3], carbon nanotubes (CNTs) [4], graphyne (GY) [5], fullerenes [6], and carbon dots (CDs) [7]. In contrast to natural bulk materials, theoretical research, practical preparation, and potential applications of carbon nanomaterials have only been in development for about one century. Nevertheless, numerous studies have demonstrated these carbon nanomaterials’ unlimited promise with retained properties of bulk materials and unique properties of low-dimensional materials. In particular, the primary fabrication of two-dimensional (2D) graphene monolayers triggered an unprecedented graphene “gold rush” while opening the door to the two-dimensional materials (2DM) system, which is a historic step for the carbon nanomaterial system [8].
In this chapter, we will take graphene, CNTs, and GY as representative carbon nanomaterials and illustrate their fabrication methods, essential properties, and applications through recent frontier research findings.
1.2 Fabrication of Carbon Nanomaterials
1.2.1 Graphene
Graphene is a 2D building material for all other dimensions of sp2-hybridized carbon materials, which can be transferred to zero-dimensional buckyballs (fullerenes), one-dimensional (1D) nanotubes, and 3D graphite (Figure 1.1) [8]. Although it was not until the early twenty-first century that graphene was isolated from bulk graphite, the concept of “graphene” or “monolayer graphite” had been proposed in the mid-twentieth century but initially used as a theoretical model that did not exist in a free state. This concept was derived from the conventional perspective proposed by Landau [9] and Peierls [10], who argued that 2D crystals could not exist independently because of their thermodynamic instability. For a long time afterward, atomic monolayer was considered to be obtained only by epitaxy on a 3D substrate. However, due to the interaction between the substrate and 2DM, epitaxial graphene may not entirely reflect the characteristics of “monolayer graphite.” Hence, researchers are still looking for methods to prepare independent graphene [11–13].
Figure 1.1 The relationship between graphene and other carbon nanostructures.
Source: Reprinted with permission from Ref. [8]. Copyright 2007, Springer Nature Limited.
In fact, since the groundbreaking isolation of graphene monolayer [2], various top-down methods, such as micromechanical cleavage, liquid-phase exfoliation, and graphene oxide (GO) reduction, have successfully proved the independent existence of 2D crystals. Furthermore, aiming for the scalable application for the post-Moore era, bottom-up methods, such as confinement-controlled sublimation (CCS) and chemical vapor deposition (CVD), are actively used in large-scale, high-quality synthesis of graphene.
1.2.1.1 Top-down Methods
1.2.1.1.1 Dry Exfoliation of Graphene
In 2004, Novoselov et al. demonstrated the preparation of few-layer graphene (FLG) consisting of monolayer by micromechanical cleavage (repeated peeling by scotch tape) of highly oriented pyrolytic graphite [2]. This surprisingly simple method produced FLG films up to in size with stability and reliability (Figure 1.2a), and the properties of FLG are almost identical to those of theoretical studies. With convenience but low yield, the original cleavage method is suitable for research or proof of concept but not practical for large-scale applications. A layer-engineered exfoliation (LEE) technique was introduced by Moon et al. for large-area, layer-controlled graphene exfoliation (Figure 1.2b) [14]. In the LEE process, bulk natural graphite was firstly cleaved on adhesive tape, and then a selective metal film was directly deposited on the graphite by e-beam evaporation. The lattice mismatch between metal and graphite produced tensile stress at the interface, which created a crack at the boundaries of graphite induced by external bending and eventually led to large-area exfoliation of graphene [15]. The thickness of exfoliated graphene was determined by the difference between the metal–graphene binding energy and interlayer binding energy of graphite , resulting in a thicker layer with a higher difference (Figure 1.2b,c) [15, 16]. Due to the slight difference between and , Au-assisted LEE graphene showed a defect-free monolayer with a large lateral size of 1 mm and could be repeatedly exfoliated from the same bulk graphite (Figure 1.2d,e). The large-area exfoliation strategy controlled by interface binding energy between metal and 2DM is also applicable in other 2DM systems, such as transition metal dichalcogenides [17].
Figure 1.2 (a) OM image of FLG produced using original micromechanical cleavage method.
Source: Reprinted with permission from Ref. [2]. Copyright 2004, The American Association for the Advancement of Science.
(b) Schematic of LEE process and the relation between and . (c) AFM profile of Co-, Ni-, and Pd-LEE graphene with the relation . (d) Low-magnification OM image of LEE graphene with millimeter-size monolayer. (e) OM images of repeated LEE graphene.
Source: Reprinted with permission from Ref. [14]. Copyright 2020, The American Association for the Advancement of Science.
1.2.1.1.2 Liquid-phase Exfoliation of Graphene
Compared with air ambience, liquid immersion can significantly reduce the van der Waals (vdW) interaction between neighboring layers of graphite. With external forces induced by sonication [18], ball milling [19], or shear mixing [20], graphene nanosheets could be easily exfoliated in the liquid phase, generally provided by water or organic solvents. High tension at the solid/liquid interface hurts solid dispersion in a liquid medium. Therefore, the selection of a liquid medium can directly determine the quality of liquid-exfoliated graphene, and solvents with a lower surface tension ( [18]) could minimize the tension at the graphene/solvent interface [21]. Furthermore, adding surfactants or intercalation particles can also weaken the vdW interaction of graphite.
Supercritical fluid (SCF) is a special substance with a temperature and pressure above the critical point , where liquid and gas phases cannot be distinct (Figure 1.3a). SCFs have specific intermediate properties between liquid and gas, such as gas-like diffusivity, liquid-like solubility, and adjustable density and viscosity controlled by temperature and pressure [22, 23]. Supercritical carbon dioxide (SCCO2), with an easily accessible critical point (, [24]), is a suitable medium for graphene exfoliation owing to extremely low surface tension, shearing effect produced by complex hydrodynamics, and intercalation of CO2 molecules. Zhu et al. characterized the SCCO2-exfoliated graphene and explained the stress mechanism in SCCO2 applying on graphite (Figure 1.3b) [25]. Tangential stress could enhance the intercalation effect of SCCO2 on graphite’s vdW gap, while the direct impact of normal stress on the graphene surface could break bonds and reduce the size of graphene. The increasing pressure makes SCCO2 denser, enhancing the stress mechanism and leading to more efficient exfoliation. Longer processing time also contributes to complete exfoliation, but excessive processing will cause the agglomeration of graphene. Under the same posttreatment condition, graphene obtained at 45 MPa, 48 h showed the highest concentration with an average thickness of ~1.272 nm, which could be regarded as two or three layers (Figure 1.3c).
Figure 1.3 (a) Schematic of a pressure–temperature phase diagram showing the supercritical region.
Source: Reprinted with permission from Ref. [22]. Copyright 2023, Wiley-VCH GmbH.
(b) Demonstration of SCCO2-assisted graphene exfoliation mechanism. (c) AFM image of graphene nanosheets with centrifugation posttreatment and corresponding height profiles along different labels.
Source: Reprinted with permission from Ref. [25]. Copyright 2021, Elsevier Ltd.
1.2.1.1.3 Electrochemical Exfoliation
Electrochemical exfoliation (ECE) is still a liquid-phase intercalation method. However, the strong electric field generated by the ECE device allows the intercalation of larger ions or molecules, resulting in fast expansion and exfoliation of 2DM. A typical dual-electrode ECE device (Figure 1.4a) consists of a working electrode (usually bulk materials for exfoliation such as graphite), a counter electrode (usually a metal plate), an electrolyte, and a power source [26]. According to charge characteristics of ions involved in intercalation, ECE can be classified into two types: anodic exfoliation, in which graphite acts as an anode to attract anions, and cathodic exfoliation, in which graphite acts as a cathode to attract cations. Anodic exfoliation is usually generated in an aqueous electrolyte containing anions such as , , , OH–, and halide (Cl−, Br−, I−). Due to the positive voltage applied to graphite, anodic exfoliation provides higher productivity but inevitably leads to oxidation and defects in the exfoliated graphene [26, 27]. Mirkhani et al. proposed a high-temperature vapor reduction process to improve the morphology and electrical properties of graphene produced by anodic exfoliation [28]. Compared with thermal and microwave treatment, graphene with HI vapor treatment indicated a more compacted structure with more metallic luster and lower sheet resistance (Figure 1.4b,c). Cathodic exfoliation is a relatively mild process since the applied negative voltage could prevent graphene’s oxidation, preserving its intrinsic properties. Nonaqueous media (e.g. N-methyl-2-pyrrolidone, dimethylformamide) containing cations such as alkali ions (Li+, K+, Na+) and quaternary ammonium ions (e.g. tetrapropylammonium, tetrabutylammonium) are commonly used for cathodic exfoliation of graphene. Most primary, secondary, and tertiary alkylammonium cations are unsuitable for cathodic exfoliation because of the instability under electrochemical potential. On the contrary, fully substituted quaternary ammonium ions can remain stable in this process [29]. However, limited by intercalation efficiency, the rate of the cathodic exfoliation process tends to be lower than that of the anodic exfoliation process [30, 31].
Figure 1.4 (a) Schematic of a typical dual-electrode ECE setup.
Source: Reprinted with permission from Ref. [26]. Copyright 2024, The Royal Society of Chemistry.
(b) Thickness and (c) sheet resistance of electrochemically exfoliated graphene (EEG) and reduced graphene (rG) by HI, thermal (Th), and microwave (MW) treatment.
Source: Reprinted with permission from Ref. [27]. Copyright 2021, American Chemical Society.
1.2.1.1.4 Reduced Graphene Oxide
Reduced graphene oxide (rGO) is graphene obtained through the chemical oxidation of bulk graphite and subsequent reduction of GO. GO is typically synthesized by Brodie [32], Staudenmaier [33], or Hummers [34] method, which treats the graphite with a potent oxidizing agent (potassium chlorate or permanganate) in a robust acid environment (nitric acid or sulfuric acid) to promote interlayer spacing enlargement and conversion to GO. GO can be easily reduced to rGO by reaction with hydrazine or thermal annealing. However, using strong acids and oxidizing agents is undesirable for the mass production of rGO but also causes considerable structural defects and much residual oxygen in rGO [13, 35]. Tao et al. developed a novel catalyst-assisted exfoliation method for high-conductivity–dispersibility graphene (HCDG) with large lateral size (Figure 1.5a) [36]. The edge-oxidized graphite flake (eoGF) was produced through controllable oxidation, which differs from the conventional oxidization process. After the intercalation of the Fe3+ ion, eoGF was finally immersed in an H2O2 solution and exfoliated into graphene sheets by the Fe3+-catalyzed decomposition of H2O2. The generated O2 weakened the vdW interaction between eoGF layers without interior oxidization, which can be demonstrated in (002) characteristics of graphene determined using X-ray Diffraction (XRD), high-resolution transmission electron microscopy (Figure 1.5b,c), and localized, low-level oxidization determined using Raman spectra and X-ray Photoelectron Spectroscopy (XPS) (Figure 1.5d,e).
Figure 1.5 (a) Schematics of catalyst-assisted exfoliation of HCDG. (b) XRD patterns of HCDG, FeCl3-eoGIC, eoGF, and pristine graphite. (c) Transmission electron microscopy (TEM) image and selected area electron diffraction (SAED) pattern of the HCDG basal plane. (d) Raman spectra of HCDG and pristine graphite. (e) C1s spectra of XPS and deconvoluted peaks of HCDG.
Source: Reprinted with permission from Ref. [36]. Copyright 2020, Wiley-VCH GmbH.
1.2.1.2 Bottom-up Methods
1.2.1.2.1 Epitaxial Graphene on Silicon Carbide
As early as 1975, van Bommel et al. discovered the sublimation of silicon from SiC lattice and nucleation of a graphene layer in ultrahigh vacuum at ~800°C [37]. Since the graphene layer was proved to be decoupled from SiC [38], graphene prepared using SiC high-temperature sublimation process was also called epitaxial graphene (epigraphene, EG). Zhao et al. demonstrated a quasi-equilibrium annealing method in which semiconductor epigraphene (SEG) is grown on macroscopic, atomically flat SiC terraces [39]. A CCS furnace with a small leak was designed for SEG growth (Figure 1.6a) [39, 40], in which graphene’s growth rate depended on silicon atoms’ escape rate. When the Si face of one SiC chip (acting as the seed) was placed oppositely to the C face of the other (acting as the source) in a CCS furnace at high temperature (>1600°C), due to the quasi-equilibrium between source and seed, subsequent step flow and step bunching occurred and eventually formed large SEG-coated (0001) facets on Si face (Figure 1.6b,c) [41]. This process differed from the depletion of the Si face in the conventional CCS process, with two Si faces placed oppositely. In addition, the SEG-covered (0001) facets were more stable than any other SiC facets, implying that wafer-level growth of single-crystal SEG should be possible in principle. Characterization of SEG showed a bandgap of 0.6 eV and ordered covalent bonds to SiC substrate, while there was no evidence of graphene on SiC (“SEG” and “graphene” cannot be equated). The electrical properties of SEG are described in Section 1.3.1.1.
Figure 1.6 (a) Schematic of a CCS furnace with two SiC chips inside a graphite crucible. (b) Schematic of two SiC chips stacked with the C face of the source chip facing the Si face of the seed chip. (c) Composite electron microscope image of a 3.5 mm × 4.5 mm SiC wafer with 80% SEG coverage. The localized area shows the contrast between SiC and SEG.
Source: Reprinted with permission from Ref. [39]. Copyright 2024, Springer Nature Limited.
1.2.1.2.2 Chemical Vapor Deposition
CVD is a widely used process for the deposition of various kinds of material in microelectronics, such as metals, elemental or compound semiconductors, gate oxides, dielectrics, etc. In the graphene CVD process, gaseous precursors like hydrocarbons are primarily used for deposition, while solid [42, 43] and liquid [44] precursors are also feasible. Transition metal is a well-known substrate for graphene CVD, acting as a catalyst for dehydrogenating hydrocarbon precursors. Nucleation and growth of graphene ensue after the dehydrogenation process, and growth mechanisms on metal substrates can be divided into two categories according to the solubility of carbon in a specific metal. Ni and Cu are the two most commonly used metal substrates for graphene CVD, corresponding to the segregation growth mechanism on metals with higher carbon solubility and the surface growth mechanism on metals with lower carbon solubility. The segregation mechanism on Ni tends to grow into multilayer graphene, in which the thickness, crystalline state, and defect of Ni substrate will influence the morphology of graphene [45, 46]. The surface mechanism on Cu refers to a self-limiting and robust reaction process, which is more favorable for monolayer graphene growth [47]. However, due to the misorientation among different initial nuclei, the merging of crystal domains originating from these nuclei can only finally form polycrystalline graphene. Two strategies have been derived from this issue for the growth of large-scale, monocrystalline graphene on Cu: one is to reduce the nucleation density and maximize the expansion of a single nucleus, and the other is to achieve the perfect coalescence of domains by controlling the consistent orientation of multiple nuclei.
Through elaborate condition control and optimization, bilayer graphene (BLG) with AB (Bernal) stacking can be further produced on Cu. Zhang et al. achieved rapid CVD growth of large-area continuous BLG on Cu foil by introducing trace CO2 (Figure 1.7a) [48]. With the assistance of CO2, ~50% BLG coverage could be achieved within 10 min based on the whole-covered monolayer graphene (MLG), and continuous BLG was obtained within 20 min. The growth mechanism of BLG was investigated by isotopic labeling with alternating introduction of and , which demonstrated that the second-layer graphene grew below the first layer and showed concentric rings composed of alternating and under Raman intensity mapping (Figure 1.7b–d). The introduced CO2 etched the first-layer graphene to form point defects, providing diffusion sites for the carbon source to form the second layer (Figure 1.7a), which differs from the conventional self-limiting growth mechanism on Cu. The AB-stacking structure was dominant in BLG, accounting for as high as 61% on Cu (100)-dominated polycrystalline Cu foils and 100% on super flat epitaxial single-crystal Cu (111) on annealed c-surface sapphire (Figure 1.7e,f). The CO2-assisted strategy also showed high compatibility in submeter-scale (0.3 m × 0.1 m) substrates and roll-to-roll mass production, achieving more than 90% coverage of BLG.
Figure 1.7 (a) Schematic of CO2-assisted BLG growth through the alternating supplement of and . (b) Schematic of alternating and pattern of graphene. (c) OM image and (d) Raman intensity mapping of G-band of half BLG domain. (e, f) Distribution of twist angles based on SAED patterns of the BLG grown on (e) polycrystalline and (f) single-crystal Cu.
Source: Reprinted with permission from Ref. [48]. Copyright 2023, Springer Nature Limited.
1.2.2 Carbon Nanotubes
CNT is a type of seamless tubular graphite structure first prepared by Iijima in 1991 [4]. Although such 1D cylindrical tubular structure was rare in inorganic crystals at that time, Iijima predicted that structure engineering of carbon nanomaterials would be possible at much larger scales than fullerenes, which was confirmed by the first isolation of graphene in 2004 [2, 4]. According to the thickness of walls, CNTs can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). A SWCNT with a specific diameter can be regarded as the structure formed by the rotation and enclosure of a GNR monolayer around a particular vector, which can be described by a pair of chiral indices (Figure 1.8) [49]. The relationship between and values reflects the configuration of SWCNTs [50, 51]. To be specific, SWCNTs with indices and are called zigzag and armchair, which are two types with high symmetry. For , the indices refer to a type of SWCNTs with chiral isomers. Chirality has an essential influence on the electrical properties of SWCNTs [52], which will be discussed in Section 1.3.2.
Figure 1.8 (a) Chiral vector of a SWCNT in the unfolded 2D graphene layer. The unit vectors and , and chiral angle are also shown. (b) Possible chiral indices with ranging from 0° to 30°. The circled dots and dots denote each SWCNT’s metallic and semiconducting behavior.
Source: Reprinted with permission from Ref. [49]. Copyright 1992, American Institute of Physics.
The main synthetic methods of CNTs are arc discharge (AD), laser ablation (LA), CVD, etc., which allow controlling and adjusting the morphology and parameters of CNTs by optimizing the growth conditions. In addition, since transition metal catalysts are necessary for the methods mentioned previously, the rational design of catalysts is also an effective strategy for controlled synthesis of CNTs [53].
1.2.2.1 Arc Discharge
The phenomenon of ADis usually generated by the approach of two electrodes under an inert ambiance. This principle was initially applied for the preparation of fullerenes, while CNTs were occasionally synthesized in this way by Iijima [4]. For AD synthesis of CNTs, graphite is used as both electrodes and high-temperature (above 1500°C) plasma is generated by the arc under a large current, which causes the sublimation of carbon atoms from the anode and coagulation into CNTs on the cathode (Figure 1.9a) [54]. Therefore, the anode graphite is continuously consumed and designed to be mobile to maintain the optimum distance between two electrodes. However, AD-CNTs are often mixed with carbon-based by-products (e.g. amorphous carbon, fullerenes) and residual catalyst particles, which require further purification. Ribeiro et al. proposed a cyclic protocol of thermal oxidation and HCl solution reflux to purify AD-CNTs [55]. Each cycle was designed to oxidize and dissolve metal particles on the walls of CNTs and remove disordered or defect-rich carbon nanostructures, achieving an impurity removal efficiency of 66−75%. Moreover, due to the high ambient temperature, AD-CNTs always reveal an excellent crystallinity and straight morphology (Figure 1.9b) [54]; yet, elaborate control on parameters of CNTs, such as diameter, length, and chirality, is challenging to achieve by this method.
Figure 1.9 (a) Schematic of CNT synthesis using AD. (b) High-resolution TEM image of MWCNTs synthesized using AD.
Source: Reprinted with permission from Ref. [54]. Copyright 2022, Elsevier Ltd and Techna Group S.r.l.
1.2.2.2 Laser Ablation
Similar to AD, the LA method for the synthesis of CNTs also involves the sublimation process of carbon atoms, which is dominated by a high-power laser under a high temperature (~1200°C) (Figure 1.10a) [56]. Unlike the intermittent discharge process of AD, LA enables a continuous and efficient synthesis of CNTs with no requirement for the conductivity of carbon targets [56, 57]. Conventional LA process of CNTs used graphite target as a carbon source, while Chen et al. recently reported high-yield synthesis of SWCNT bundles using low-graphitized coal as a carbon source (Figure 1.10b) [57]. Despite complex carbon components and various impurity elements in coal, it is more easily ablated than graphite under the same LA conditions, resulting in a higher carbon evaporation rate. LA-SWCNTs synthesized from coal present an ideal distribution of diameter (~1.1–1.3 nm at 1198 K) for electronic applications, which can be fabricated as the active layer of carbon nanotubes field effect transistors (CNT-FETs) (Figure 1.10c). However, it should be pointed out that the high-power laser in the LA setup prohibits its large-scale application to CNT synthesis [53].
Figure 1.10 (a) Schematic of CNT synthesis using LA. (b) TEM image of SWCNTs with marked catalyst. (c) Length distribution of SWCNTs sorted using PCz (poly[9-(1-octylonoyl)-9H-carbazole-2,7-diyl]).
Source: Reprinted with permission from Ref. [57]. Copyright 2023, The Royal Society of Chemistry.
1.2.2.3 Chemical Vapor Deposition
Due to the unacceptable condition of high temperatures in the AD and LA process, CVD is a suitable and mature alternative for synthesizing CNTs. On the one hand, according to the physical state of metal catalysts, typical substrate-supported catalyst CVD (SCCVD) mechanisms of CNTs can be classified into either vapor–solid–solid (VSS) or gas–liquid–solid (VLS). In the VSS growth mechanism, the solid catalyst usually maintains a relatively stable morphology, and the precipitation of carbon atoms only happens on the catalyst’s surface. The structure and components of the catalyst can be manipulated to narrow the diameter and chirality distribution of CNTs [58, 59]. In the VLS mechanism, the liquid state of the catalyst will lead to the dissolution of carbon and the movement of the catalyst droplet. Hence, the chirality of the synthesized CNT is randomly distributed.
On the other hand, as CVD growth progresses, the catalyst may eventually be located at the base or the tip of CNTs. VSS growth of CNTs may follow a base- or tip-growth mechanism, which is related to the strength of the interaction between catalyst metal and substrate, and the growth direction (vertical or parallel to substrates) of CNTs can be adjusted by plasma or airflow [60, 61]. VLS growth of CNTs is generally regarded as a tip-growth mechanism [62], as carbon atoms are only precipitated at the migrating catalyst droplet.
Different from metal catalyst pre-deposition in SCCVD methods, floating catalyst CVD (FCCVD) introduces catalyst precursor and gaseous carbon source together into the reaction chamber, followed by catalyst formation, carbon nucleation, and CNT growth [63, 64]. FCCVD-CNTs are usually floating in the chamber, which can be blown out with airflow and collected easily [65]. However, due to the short reaction time in the flowing atmosphere, CNTs are blown out without sufficient growth, exhibiting short lengths and random alignment. Jiang et al. proposed a substrate interception and orientation strategy (SIDS) to fabricate high-density ultralong CNT arrays by combining the so-called “kite-mechanism” (tip-growth mechanism) of ultralong CNTs with an improvement of FCCVD (Figure 1.11a,b) [66]. A substrate was placed horizontally into the center of the FCCVD chamber and used to intercept and reorient the floating short CNTs, which is absent in conventional FCCVD (Figure 1.11c,d). Local turbulence at the edge of the substrate caused one end of the CNTs to be anchored to the vertical step of the substrate, which sequentially followed the kite-like growth mechanism with an adequate flowing supply of carbon source (Figure 1.11e–g). Conventional growth of ultralong CNTs was based on VSS methods. In this case, the tip-growing CNTs initially need to overcome external forces such as vdW and drag forces to grow vertically without lying on the substrate [61, 67]. Actually, these dead CNTs account for a considerable portion of the total, resulting in a low catalyst utilization rate and CNT density [67].
Figure 1.11 (a) Schematic of ultralong CNT synthesis using SIDS. (b) CFD simulation of the velocity distribution and streamlines adjacent to the substrate. (c–e) Schematics of (c) interception, (d) reorientation and growth, and (e) the resultant array of ultralong CNTs during the FCCVD process with SIDS. (f, g) Scanning electron microscopy (SEM) image of the high-density ultralong CNT arrays with different scale bars.
Source: Reprinted with permission from Ref. [66]. Copyright 2023, American Chemical Society.
1.2.3 Graphyne
GY is a type of 2D carbon nanostructure predicted by Baughman et al. in 1987, in which sp- and sp2-hybridized carbon atoms coexist [68]. In GY structures, benzene rings are connected by acetylene bonds and form a hexagonal symmetric network, resulting in four types of chemical bonds with different lengths between carbon atoms: (i) bonds in benzene rings; (ii) acetylene bonds; (iii) bonds between benzene ring and acetylene bond; and (iv) bonds between two acetylene bonds [69]. According to the number of acetylene bonds between two benzene rings, GY family can be typically classified into GY , graphdiyne (GDY) , graphtriyne , and graphtetrayne , or uniformly indicated by GY- (Figure 1.12) [70]. Among them, GDY is one of the most widely studied GY structures, which was first synthesized by Li et al. in 2010 [5].
Figure 1.12 GY containing different numbers of inserted acetylene bonds.
Source: Reprinted with permission from Ref. [70]. Copyright 2023, American Chemical Society.
GDY synthesis largely depends on the formation of diacetylene bonds, which can be achieved through an alkyne coupling reaction between two aryl monomers. In the early stages of exploring the synthesis of GDY, studies mainly focused on synthesizing several substructures of GDY or nanoGDY due to the difficulty in controlling the conditions for large ordered couplings [71]. The large-area synthesis of GDY was not extensively studied until Li et al. introduced the cross-coupling reaction of hexaethylbenzene (HEB) and demonstrated the obtained nanoscale GDY films [5]. According to the physical state of the substrate, GDY synthesis can be mainly classified into liquid-phase and solid-phase synthesis.
1.2.3.1 Liquid-phase Synthesis
The initial synthesis of GDY developed by Li et al. was carried out in a liquid-phase environment provided by tetrahydrofuran [5]. The cross-coupling reaction of HEB was driven by the release of trace Cu(II) ions from the Cu foil in an alkaline environment provided by pyridine, which ultimately formed a GDY thin film on Cu. Hence, the Cu foil served a dual role as catalyst and substrate [72]. Li et al. synthesized crystalline GDY (6L, ~2.19 nm) through a modified Glaser–Hay coupling reaction of HEB and directly observed the rhombohedral (ABC) stacking in GDY by low-voltage TEM (Figure 1.13a,b) [73]. Direct characterization of few-layered GDY crystals is difficult due to structural fragility. Apart from growth at the liquid/solid interface, GDY can also grow at liquid/liquid and liquid/gas interfaces [74, 75]. Liquid/liquid interface is usually provided by water and water-immiscible organic solvents. During the reaction process, the catalyst in the aqueous phase and monomers in the organic phase undergo a coupling reaction at the interface and gradually grow into a suspended GDY layer, which can be easily transferred to other substrates using Langmuir–Schäfer method (Figure 1.13c,d). The stability of the liquid/liquid interface can effectively avoid the random interaction between monomer and catalyst [74]. The thickness of GDY can be adjusted by changing the concentrations of monomer and catalyst. GDY modification can be achieved by adding metal ions to the aqueous phase to form metal-GDY composite structures [76].
Figure 1.13 (a) Experimental SAED pattern and (b) schematic stacking mode of ABC-stacked GDY.
Source: Reprinted with permission from Ref. [73]. Copyright 2018, Tsinghua University Press and Springer-Verlag GmbH Germany.
(c, d) Multilayer HgL1 (L1=tris(4-ethynylphenyl)amine) nanosheets (c) at the liquid/liquid interface and (d) on a slide glass.
Source: Reprinted with permission from Ref. [75]. Copyright 2021, Wiley-VCH GmbH.)
Besides interface-mediated synthesis, GDY can be synthesized directly in the liquid phase through coupling reaction. He et al. proposed a mild, convenient, and tunable one-pot method for the synthesis of GDY through Pd-catalyzed decarboxylation coupling reaction (Figure 1.14a) [77]. The synthesized GDY was characterized by typical 2D structure, high degree of crystallinity, and exhibited localized folding behaviors (Figure 1.14b,c). Li et al. proposed a space-confined synthesis method using MXene as a template for precise control of GDY’s thickness and long-range ordering (Figure 1.14d) [78]. The sub-nanometer-sized gap between MXene layers allows monomers to be intercalated and coupled in situ and effectively constrains the out-of-plane growth or vertical stacking of GDY, ultimately yielding ML-GDY within MXene layers (Figure 1.14e). By liquid-phase exfoliation, 2DM mixtures containing free-standing ML-GDY with a lateral size distribution of could be obtained, in which the monolayer thickness and ordered in-plane structure of GDY were preserved (Figure 1.14f). Yan et al. directly exfoliated GDY bulk materials into monolayer or multilayer sheets by the assistance of Li2SiF6 inorganic salt (Figure 1.14g) [79]. The most challenging aspect of GDY exfoliation is the potential introduction of structural defects or internal oxidation, which may affect the properties and application value of the eGDY. The noncovalent interactions generated by and the intercalation of small-radius Li+ together contributed to the efficient exfoliation and the preservation of GDY’s original crystal structure without generating additional oxides (Figure 1.14h,i).
Figure 1.14 (a) Schematic of GDY synthesis by decarboxylation coupling reaction. (b) The SEM image and (c) SAED pattern of GDY.
Source: Reprinted with permission from Ref. [77]. Copyright 2023, The Royal Society of Chemistry.
(d) Schematics of MXene template strategy for multilayer graphdiyne (MLGDY) synthesis. (e) High Angle Angular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) and Integrated Differential Phase Contrast Transmission Electron Microscopy (iDPC-STEM) of GDY-MXene structure along the [001] axis. The image intensity profile from the marked area is shown below. (f) TEM image and SAED pattern (inset) of free-standing monolayer GDY (ML-GDY).
Source: Reprinted with permission from Ref. [78]. Copyright 2023, Wiley-VCH GmbH.
(g) Salt-assisted exfoliation mechanism of GDY by three steps. Step I: adsorption of anions and cations on GDY; Step II: intercalation of cations into the charged interlayer; Step III: dispersed GDY in salt solution. (h) Wide-range XPS and (i) C1s spectra of bulk GDY powder and exfoliated GDY (eGDY).
Source: Reprinted with permission from Ref. [79]. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
1.2.3.2 Solid-phase Synthesis
Metal-catalyzed, self-limited CVD is also suitable for GDY synthesis (Figure 1.15a,b). Although Cu maintains a high catalytic activity in the liquid-phase synthesis of GDY, in the case of CVD, Ag is more advisable than Cu as a catalyst and substrate for coupling. Unlike graphene CVD using gaseous carbon precursors such as CH4, the CVD process of GDY must be kept at a low temperature to prevent the destruction of the aryl precursor [80]. Liu et al. obtained large ML-GDY films on Ag foils with hexaethynylbenzene (HEB) or 1,3,5-triethynylbenzene (TEB) as precursor at 150°C, which could be transferred by a standard PMMA process for further characterization (Figure 1.15c,d) [80]. Mechanical forces such as ball milling have also been proven to be an effective option for driving solvent-free, alkyne-based coupling reactions without catalyst assistance. Li et al. prepared pure GDY by ball-milling-driven reaction of calcium carbide (CaC2) and hexabromobenzene and confirmed the 1 : 1 sp/sp2-hybridized carbon ratio of GDY by characterization (Figure 1.15e,f) [81]. Previously, the group had successfully prepared hydrogen-substituted GDY by mechanochemical coupling of CaC2 and 1,3,5-tribromobenzene [82].
Figure 1.15 (a, b) Schematics of (a) CVD system for GDY growth on Ag and (b) the surface growth process using HEB as a precursor. (c) Opticalmicroscopy (OM) and (d) Atomic force microscopy (AFM) images of GDY film transferred on SiO2/Si substrate. Inset in (d): height profile along the dashed line. Scale bars: (i) and (ii) .
Source: Reprinted with permission from Ref. [80]. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(e) Ball-milling-driven reaction for GDY preparation. (f) SAED pattern of as-prepared GDY.
Source: Reprinted with permission from Ref. [81]. Copyright 2018, Elsevier Ltd.
1.3 Properties and Applications of Carbon Nanomaterials
1.3.1 Graphene
According to the 1995 IUPAC Recommended Terminology, the term “graphene” was initially used to describe a single carbon layer with a graphitic structure, which should be used only when discussing the reactions, structural relations, or other properties of individual layers [83, 84]. In fact, with the increase in the number of stacked graphene monolayers, band structure evolves rapidly and approaches the 3D limit of graphite at 10 layers [8, 85]. Thus, the meaning of “graphene” extends from monolayer to 2D stacked systems of up to 10 layers, and it is crucial to make a distinction when focusing on such materials. When discussing specific properties, especially electrical properties, it is necessary to classify graphene into MLG, BLG, trilayer (TLG), and multilayer (<10 layers), as the difference in properties due to the variation in the number of layers will lead to different applications for these graphene structures.
1.3.1.1 Electrical Properties and Applications
Under the tight-binding approximation, intrinsic MLG shows a linear dispersion relation in its Brillouin zone at the K and K′ points (Dirac points) [86, 87]. MLG’s conduction and valence bands intersect at Dirac points; hence, it behaves as a zero-bandgap semimetal. Due to the ballistic transport properties of carriers, graphene can exhibit high mobility (theoretical maximum of ~200 000 cm2/(V·s) at room temperature (RT) [88]), while the lack of bandgap becomes a major stumbling block for carrier modulation, which creates difficulties in the application of graphene field effect transistors (GFETs). Therefore, effective carrier modulation methods can significantly improve the performance of GFETs and highlight the advantage of high mobility. Zheng et al. introduced a graphene strain-effect transistor (GSET), which restricts and recovers current conduction through reversible nanocrack in the source/drain metal contacts (Figure 1.16a,b), resulting in a vast ratio of over 107 [89]. When a back-gate voltage exceeding the switching threshold was applied to GSET, the piezoelectric lead zirconate titanate back-gate dielectric strained on the Ni/Au contacts and eventually switched off the GSET. The contrary ambipolar characteristics (OFF–ON–OFF) of GSET were quite different from the conventional ambipolar characteristics (ON–OFF–ON) (Figure 1.16c). Furthermore, the switching threshold voltages ( and ) of GSET tended to stabilize after a 17-strain cycling process (Figure 1.16d), suggesting a reversible strain mechanism. In the ON-state ( and |), GSET exhibited linear output characteristics, indicating the semimetallic nature of graphene.
Figure 1.16 (a) 3D schematic of the post-strain cycling process of GSET. (b) SEM image of the strain-induced nanogap across Ni/Au contact. (c) Transfer characteristics of GSET at the 17th strain cycle. (d) Switching threshold voltages ( and ) of GSET in 17 strain cycles.
Source: Reprinted with permission from Ref. [89]. Copyright 2023, American Chemical Society.
(e) Schematics of GWG-FETT with the crystalline structure of each layer shown in the enlarged view. (f) OM image of GWG-FETT. (g, h) Corresponding transfer curves of GWG-FETT at (g) and (h) . (i–l) Energy band alignment diagrams at (i, j) and (k, l) under zero and positive .
Source: Reprinted with permission from Ref. [90]. Copyright 2022, American Chemical Society.
vdW heterostructure combines different 2DM through vdW interactions, offering preparation flexibility and good interfacial contacts [91]. Bai et al. achieved complementary properties between graphene and WS2 by preparing highly tunable field-effect tunneling transistors (FETTs) with vertically stacked graphene-hBN-graphene-WS2-graphene heterostructure (Figure 1.16h) [90]. Multilayer WS2 has a suitable indirect bandgap of 1.4 eV [92], which enables the alignment between the Fermi level of graphene and the conduction band minimum of WS2 and contributes to efficient carrier tunneling. In graphene-WS2-graphene (GWG) heterostructure, the barrier shape was modulated by the bias voltage between the bottom (B-Gr) and top graphene layer (T-Gr), which switched the tunneling mechanism between direct tunneling under a small and Fowler–Nordheim tunneling under a large (Figure 1.16i–k). GWG-FETTs also changed the operating states through between n-type and bipolar complementary metal oxide semiconductor-like tunneling devices (Figure 1.16). Moreover, the thermionic emission mechanism was proved to contribute significantly to the on-current of FETTs (Figure 1.16l), as evidenced by high ratios of 1.5 × 106 and 5 × 108 at RT (300 K) and low temperature (5 K), respectively, under small .
The switching performance of GFETs can also be improved by modifying the band structure of graphene. In the pioneering report by Novoselov et al., the degree of energy band overlap varied in FLGs with different thicknesses, and the thinnest FLG sample was considered to possess a zero bandgap [2]. The overlap could be used to account for the inversion of Hall coefficients in FLG with the variation of gate voltage (Figure 1.17a), suggesting that FLG could be transformed from mixed-carrier material to either fully electronic or entirely hole conductor. Li et al. introduced a bandgap by changing the hybridization form of carbon atoms in graphene (Figure 1.17b) [93]. A reversible hydrogenation process realized this transition in a highly concentrated hydrogen-ion electrolyte, in which the electric field generated by induced the hydrogen adsorption and desorption on GFETs. When the positive increased above the hydrogenation potential, the graphene lattice became highly activated and transformed into sp3-hybridized insulating hydrogenated graphene. As was swept back to negative, the gradual increase in indicated the recovery of graphene properties due to dehydrogenation. This reversible, hydrogen-induced conductor–insulator transition was completely observed in MLG (Figure 1.17d), BLG, and TLG with stable curves. The hydrogenation strategy of graphene showed reliable gate-controlled switching capability, remaining operative without any degradation of after 1 million (1M) switching cycles of MLG-FETs (Figure 1.17c). Zhao et al. p-doped the SEG devices by charge transfer of adsorbed oxygen to exhibit the intrinsic transport properties of SEG on SiC (Figure 1.17e; see Section 1.2.1.2 for SEG growth method) [39]. With the gradual temperature increase in the 100–300 K range, the conductivity and Hall mobility of the samples also showed an increasing trend with maximum mobility of 5500 cm2/(V·s) (Figure 1.17f). The mobilities of S2, S3, and S4 increased rapidly and then tended to saturate with the temperature increase, which reflected the transition of the transport mechanism from localized defect states in the bandgap to high-mobility band transport, confirming the semiconductor nature of SEG. The ratio of ~106 and SS of ~60 mV/dec obtained by the density of states calculations demonstrated that the properties of SEG were sufficient for digital electronic applications.
Figure 1.17 (a) Hall coefficient of FLG versus at .
Source: Reprinted with permission from Ref. [2].
(b) Schematic of hydrogenation process between the graphene lattice and H+ ions. (c) (left) measurement after 1M switching cycles with periodically changed (right) between −0.5V and 2.4V. (d) Three consecutive cycles of measurements in MLG versus with fixed .
Source: Reprinted with permission from Ref. [93]. Copyright 2021, Springer Nature Limited.
(e) Schematic of SEG p-doping process by charge transfer. (f) Hall mobilities of different SEG devices of S1–S8 versus temperature.
Source: Reprinted with permission from Ref. [39]. Copyright 2024, Springer Nature Limited.
1.3.1.2 Optoelectronic Properties and Applications
The unique properties of graphene have likewise attracted extensive interest in optoelectronics, prompting the construction of a range of graphene-based optoelectronic devices such as photodetectors (PDs), modulators, and hybrids. However, a number of factors, such as low light absorption (~2.3%), large dark currents arising from zero bandgap, and difficulty of separation and collection of ultrafast photocarriers, have limited further development of graphene in optoelectronics [94]. Koepfli et al. adopted a metamaterial integration strategy to maximize light absorption and photogenerated carrier extraction and realized high bandwidth (>500 GHz) and wide operating window (from <1400 nm to >4200 nm) for metamaterial graphene PD [95]. The metamaterial absorber consisted of a relatively simple stack of metal-insulator-MLG-metal-insulator layers to achieve an almost perfect light absorption (Figure 1.18a–e) [96–98]. The dipole resonators were connected with interdigitated metal contacts, and a thin silver layer was added underneath the thicker gold contact layer on every other line, which is the key to more efficient carrier extraction (Figure 1.18f). Under 1550-nm illumination, despite the low responsivity of PD ( and ), the linear behavior of response allowed the PD to maximize the input power up to 100 mW (Figure 1.18), giving rise to high photocurrent and the highest graphene data rate of 132 Gbit/s so far. The high bandwidth of >500 GHz was mainly attributed to shorter carrier transit time (Figure 1.18), which was related to the structure design of the metamaterial. Through the direct alteration of dipole resonator length, the absorption spectra of PD revealed a tunable range of >3000 nm (Figure 1.18), having potential in both sensing and telecommunication applications.
Figure 1.18 (a) Schematic of Au-Al2O3-MLG-Ag/Au-Al2O3 stacked metamaterial layers. (b) Absorption distribution with strong confinement close to the dipole resonators. (c) Extracted photocurrent for an optical input power sweep over five orders of magnitude (from 10−3 to 102 mW). (d) Normalized frequency response of the graphene PD over a 2- to 500-GHz range. (e) Measured spectral absorption of different metamaterial dipole lengths, which shows the wavelength tunability. (f) Four example SEM images of polarization-independent design, where the colors correspond to the resonator-length scale bar shown in (e). Scale bar: .
Source: Reprinted with permission from Ref. [95]. Copyright 2023, The American Association for the Advancement of Science.
Stacking of monolayers enriches the optoelectronic applications of graphene. Construction of a homogeneous p-n junction is a common strategy to enhance the performance of graphene PDs since the bandgap of BLG could yield some optoelectronic properties beyond MLG [99]. Titova et al. demonstrated the enhancement of the sub-terahertz response of split-gate BLG p-n junction PDs by an electrically induced bandgap (Figure 1.19a,b) [100, 101
