Fabrication Of Nanoscale Devices Research Articles

Ultrawide-Bandgap Nanoscale p-n Junction Field Effect Transistors of GaN / β-Ga2O3 for High Power Operation

β-Ga2O3 is emerging as a next-generation ultrawide bandgap materials for high efficiency power devices, owing to its large direct band gap (4.85eV), high breakdown field (~ 8 MV / cm) and excellent thermal∙chemical stability. The Baliga’s figure-of-merit (BFOM) of β-Ga2O3 is 3214.1, which is superior to other materials such as GaN (846.0) or SiC (317.1). In addition to superior characteristics, the availability of large crystalline substrates with controllable carrier concentrations from semi-insulation to n-type is responsible for many of the recent improvements that will utilize β-Ga2O3 as near future power device material. However, potential of β-Ga2O3 has not yet been fully explored because of the low thermal conductivity of β-Ga2O3 and absence of p-type doped β-Ga2O3. To overcome this limitation, integration with p-type materials with excellent thermal conductivity is being studied.Although β-Ga2O3 is not a van der Waals material, β-Ga2O3 can be mechanically exfoliated into a thin layer due to the large anisotropy of the unit cell from the single crystal substrate. The mechanically exfoliated flakes with high crystallinity have no significant strains due to lattice mismatch with the underlying substrate, which has great advantages in integration with other substrates and miniaturization of devices. Based on these advantages, various studies have been conducted to fabricate nanoscale devices that utilize the excellent properties and overcome the disadvantages of β-Ga2O3, through integration of exfoliated flakes with p-type substrates such as diamond and SiC. In our work, nanoscale junction field effect transistors (JFETs) were fabricated by utilizing p-n junction between β-Ga2O3 flakes and p-type GaN substrate.β-Ga2O3 / GaN JFETs were fabricated by using the β-Ga2O3 flakes exfoliated from single-crystal bulk substrate. Dielectric layer between GaN substrate and β-Ga2O3 metal electrode were defined by hydrogen silsesquioxane (HSQ), followed by e-beam patterning. β-Ga2O3 flakes were precisely transferred on HSQ pattern by PDMS-assisted dry transfer method. Ohmic metal of GaN (Ni/Au) and β-Ga2O3 (Ti/Au) was deposited by e-beam evaporator, followed by rapid thermal annealing to confirm ohmic contact of GaN and β-Ga2O3, respectively. Fabricated β-Ga2O3 / GaN JFETs showed outstanding electric properties with high rectification ratio between p-n junction of GaN and β-Ga2O3. Fabricated devices showed excellent n-type DC output and transfer characteristics even after few weeks, which shows excellent long-term air-stability. Robustness and stability of fabricated devices under harsh conditions were confirmed by operating devices under various temperature. Thermal distribution and electric field simulation of fabricated JFETs were performed to confirm the effect of GaN substrate in thermal issues. In this study, we present the way to integrating β-Ga2O3 devices with p-GaN, resolving issues of thermal conductivity and p-type doping without lattice mismatch or defects. The details of our work will be discussed in the conference.

Atomic layer deposition of a uniform thin film on two-dimensional transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDs) is one of the promising materials for future electronics since they have, not only superior characteristics, but also a versatility that conventional materials do not have with a few nanometer thickness. One of the prerequisites for applying these materials to device fabrication is to deposit an ultrathin film below 10 nm with excellent uniformity. However, TMD has quite a different surface chemistry and is fragile to external conditions compared to conventional materials. Thus, thin film deposition on 2D TMD with excellent uniformity using conventional deposition techniques is quite challenging. Currently, the most adequate deposition technique for sub-10 nm-thick film growth is atomic layer deposition (ALD). A thin film is formed on the surface by the reaction between chemical and surface species based on the self-limiting growth manner. Owing to its unique and superior growth characteristics, such as excellent uniformity and conformality, ALD is an essential deposition technique for nanoscale device fabrication. However, since 2D TMD has a lack of reaction sites on the surface, various studies have reported that ALD on 2D TMDs surfaces without any treatment showed an island growth mode or formation of clusters rather than continuous films. For this reason, recent studies have been focused on the deposition of an ultrathin film on 2D TMDs with excellent uniformity. For a decade, there have been various approaches to obtain uniform films on 2D TMDs using ALD. Among them, the authors focus on the most frequently researched methods and adsorption control of chemical species by modifying the process parameters or functionalization of new chemical species that can assist adsorption on the chemically inert 2D TMD surface. In this review, the overall research progress of ALD on 2D TMD will be discussed which would, in turn, open up new horizons in future nanoelectronics fabrication using 2D TMDs.

Preparation, doping modulation and field emission properties of square-shaped GaN nanowires

GaN nanomaterials, as one of the most important third-generation semiconductor materials, have attracted wide attention. In this study, GaN nanowires with square cross section were successfully prepared by microwave plasma chemical vapor deposition system. The diameters of nanowires are from 300 to 500 nm and the lengths from 15 to 20 μm. The results show that the cross section of nanowires could be transformed from triangle into square by adjusting the ratio of Mg to Ga in source materials. X-ray diffraction(XRD)result indicate that the structure of GaN nanowires are agree with the hexagonal wurtzite. X-ray photoelectron spectroscopy (XPS) rusult show that a certain amount of Mg and O impurities incoporated in the square-shaped GaN nanowires. Transmission electron microscopy (TEM) result suggested that square-shaped GaN nanowires had high crystallinity with a growth direction of [<inline-formula><tex-math id="M500">\begin{document}$0\bar 110$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200445_M500.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200445_M500.png"/></alternatives></inline-formula>]. The ratio of source materials- and time-depented growth mechanism was also studied. It was suggested that the transformation of the cross section from triangle to square structure should be derived from the growth mechanism change from vapor-liquid-solid(VLS)process to vapor-solid(VS)process. The doped Mg increased the growth rate of the nanowires sidewalls, which led to a symmetrically growth of GaN nanowires along the twin boundaries. GaN nanowires gradually transformed to square structure by auto-catalytic growth. Moreover, the property of field emission were further investigated. The results showed that the turn-on electric field of square-shaped GaN nanowires was 5.2 V/m and a stable field emission property at high electric field. This research provides a new method for the preparation of square-shaped GaN nanowires and a prospective way for the design and fabrication of novel nano-scale devices.

Atomic layer deposition for nonconventional nanomaterials and their applications

Abstract

Surface-induced thickness limit of conducting La-doped SrTiO3 thin films

We report on a surface-induced, insulating, electrically dead layer in ultrathin conducting La-doped SrTiO3 thin films. Systematic studies on electrical properties as a function of film thickness and La-doping levels reveal that the insulating layer has a constant thickness and traps a constant amount of electron density regardless of La-doping levels. Growing an additional capping layer on top of the La-doped SrTiO3 surface counteracts the reduced conductivity, indicating a strong relationship between the insulating layer and the surface structure. Our results emphasize the importance of surface state studies for functional oxides in the thin film limit and provide a guiding principle for the fabrication of La-doped SrTiO3-based oxide nanoscale devices.

Investigation of ma-N 2400 series photoresist as an electron-beam resist for superconducting nanoscale devices

Superconducting nanowire-based devices are increasingly being used in complex circuits for applications such as photon detection and amplification. To keep up with the growing circuit complexity, nanowire processing is moving from single layer fabrication to heterogeneous multilayer processes. Hydrogen silsesquioxane (HSQ) is the most common choice of negative-tone electron-beam resist for patterning superconducting nanowires. However, HSQ has several limitations, including an inability to be removed without a strong reagent that damages the superconducting film, making it unsuitable for multilayer fabrication. As a result, it is vital to consider alternative resists that can be removed through less harmful solvents. Here, the authors explore the use of ma-N 2400 series deep ultraviolet photoresist as an electron-beam resist for fabricating superconducting nanowire devices. They demonstrate that ma-N can be used to pattern dense lines as narrow as 30 nm and isolated features below 20 nm in width. They also examine the reproducibility of 36 identical superconducting devices by comparing their minimum dimensions and switching currents. Through this analysis, they conclude that ma-N 2400 is a suitable electron-beam resist for fabricating nanoscale devices and has the potential to expand the use of nanowire-based technologies into more advanced applications.

Applications and challenges of elemental sulfur, nanosulfur, polymeric sulfur, sulfur composites, and plasmonic nanostructures

As an abundant and biologically active element, sulfur has been applied in various fields. Through the nanotechnology method, nanosulfur is synthesized with the merits of size effects. Differentiated properties, structures, and functions of sulfur could lead to different applications. In particular, various sulfur composites are synthesized and showed satisfied performance in environmental fields. In addition, plasmonic nanomaterials possess unusual properties and potential applications in mesoscopic physics and the fabrication of nanoscale devices, which may create a new applying direction for sulfur. Undoubtedly, with the wide application, it might pose potential ecological risk and health effects, and the effective strategies for their safe applications are required. Herein, the applications and challenges of sulfur were reviewed in a line of application – risk control – safe application. Till now, although some reviews have discussed sulfur studies in some aspects, but a specific review on sulfur integrating its different sizes and forms is rarely reported. We prepared this review aiming to provide a comprehensive knowledge about sulfur. The current knowledge indicated that there lack sufficient specialized studies on sulfur, and their researching fields, environmental behaviors and fates, characterization, and plasmonic applications.

Soft High-Resolution Neural Interfacing Probes: Materials and Design Approaches.

Neural interfacing probes are located between the nervous system and the implanted electronic device in order to acquire information on the complex neuronal activity and to reconstruct impaired neural connectivity. Despite remarkable advancement in recent years, conventional neural interfacing is still unable to completely accomplish these goals, especially in long-term brain interfacing. The major limitation arises from physical and mechanical differences between neural interfacing probes and neural tissues that cause local immune responses and production of scar cells near the interface. Therefore, neural interfaces should ideally be extremely soft and have the physical scale of cells to mitigate the boundary between biotic and abiotic systems. Soft materials for neural interfaces have been intensively investigated to improve both interfacing and long-term signal transmission. The design and fabrication of micro and nanoscale devices have drastically decreased the stiffness of probes and enabled single-neuron measurement. In this Mini Review, we discuss materials and design approaches for developing soft high-resolution neural probes intended for long-term brain interfacing and outline existent challenges for achieving next-generation neural interfacing probes.

Atomic layer deposition of ZnO on MoS2 and WSe2

Atomic layer deposition (ALD) is an enabling technology for the fabrication of many nanoscale semiconductor devices. This study focuses on ALD of ZnO on the two-dimensional (2D) materials MoS2 and WSe2. Mechanically exfoliated flakes and coalesced films of MoS2 and WSe2 acted as substrates for ALD and plasma-enhanced ALD (PEALD) of ZnO at 125 °C, and were investigated by atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The MoS2 and WSe2 surfaces resisted nucleation during thermal ALD everywhere except defects, allowing for selective growth of ZnO on the surrounding SiO2 substrate for over 500 cycles. UV-O3 pre-treatment showed different results for MoS2 and WSe2. On MoS2, UV-O3 pre-treatment was effective in inducing nucleation; ZnO was deposited on different areas of the flake with different thicknesses, and coalesced films formed, unlike ZnO deposited during thermal ALD without pretreatment. On WSe2, ZnO nucleated only on certain more reactive areas of the surface that had been oxidized during treatment. PEALD was successful in growing uniform and coalesced films of ZnO on both materials but destroyed the top layer of TMD by oxidation at the conditions tested. The interface of ZnO and 2D materials might be used in future nanoscale devices, and this study shows that it may be effectively controlled using different ALD methods.

Probe assisted localized doping of aluminum into silicon substrates.

Precise control of dopant placement is crucial for the reproducible, and reliable, nanoscale semiconductor device fabrication. In this paper, we demonstrate an atomic force microscopy (AFM) probe assisted localized doping of aluminum into an n-type silicon (100) wafer to generate nanoscale counter-doped junctions within two nanometers of the silicon-air interface. The local doping results in changes in electrostatic potential, which are reported as contact potential difference, with nanoscale spatial resolution. In contrast to the literature where nano-mechanical defects in, or contaminants on, silicon substrates can result in measurable changes in the chemical potential of the near-surface, additional thermal treatment was needed to electrically activate the aluminum dopants in our current work. Unfortunately, the thermal activation step also caused the dopants to diffuse and geometric distortions in the doped area, i.e., broadening and blurring of the electrically distinct areas. The results from optimization efforts show that the "active" dopant concentration depended primarily on the thermal anneal temperature; additional AFM-tip dwell time during the aluminum implantation step had no meaningful impact on the electrical activity of the doped sites.

DNA-Powered Stimuli-Responsive Single-Walled Carbon Nanotube Junctions

Reconfigurable stimuli-responsive molecular materials play an important role in the fabrication of nanoscale systems and devices. Here, we report a bottom-up strategy for the reversible assembly of single-walled carbon nanotube linear junctions in solution. The assembly/disassembly of the nanotubes can be controlled via the intrinsic responsiveness to different stimuli of sequence-specific deoxyribonucleic acid linkers forming the junctions.

Guided Assembly of Block Copolymers in Three-Dimensional Woodpile Scaffolds.

Three-dimensional (3D) nanofabrication using the directed self-assembly of block copolymers (BCPs) holds great promise for the nanoscale device fabrication and integration into 3D architectures over large areas with high element densities. In this work, a robust platform is developed for building 3D BCP architectures with tailored functionality using 3D micron-scale woodpile structures (WPSs), fabricated by a multiphoton polymerization technique. By completely filling the spaces of the WPSs and using the interactions of the blocks of the BCPs with the struts of the WPS, well-developed 3D nanoscopic morphologies are produced. Metal ion complexation with one block of the copolymer affords a convenient stain to highlight one of the microdomains of the copolymer for electron microscopy studies but also, with the reduction of the complexing salt to the corresponding metal, a simple strategy is shown to produce 3D constructs with nanoscopic domain resolution.

(Invited) Fabrication and Characterization of Carbon-Based Nanoscale Devices: Insights and Applications

Nanoscale devices made from carbon-based materials are investigated for a variety of unique properties and features, including their promise to improve performance, decrease production costs, or provide totally new functionality relative to extant electronic devices. Because these devices have active areas composed of materials that are much thinner than those used in conventional devices, the details of the interfaces can often act to control the physics that lead to the operational device characteristics in unexpected ways. This is particularly true in molecular electronics, where design rules based on chemical intuition often fail to account for key physical aspects that dominate device behaviour. Thus, the ability to modify interfaces in a way that leads to control of device properties is a key to enabling next-generation devices based on nanoscale phenomenon. In addition, it is important to achieve an understanding of the processes that occur during carrier transport in nanoscale devices in order to obtain desirable functionality. This presentation will describe several aspects of carbon-based nanoscale devices, including fabrication and operation of molecular electronics and graphene field effect transistors (GFETs). After a discussion of some general physical principles that operate within the interfacial energy level alignment regime in molecular electoronics, a discussion of how light emission from nanoscale devices can be used to characterize them will be provided. In particular, we have used light emission from both large area molecular junctions and GFETs to understand important distance scales (elastic limits) and processes (transport and emission mechanisms). The data indicate that processes that involve hot carrier interactions with plasmonic structures in the devices lead to light emission, and that this phenomenon can be used to measure energy losses of carriers as they traverse a molecular layer. Results indicate that carriers can travel approximately 7 nm through a molecular junction before energy losses become significant, indicating that elastic transport is achieved for thin layers, but a transition in mechanism occurs for thicker films. The characteristics of the energy losses are reported for several different structures, providing insights into the nature of the transport in molecular electronics. Light emission from GFETs, on the other hand, appears to follow a novel mechanism that involves the excitation of plasmons in the graphene by hot carriers followed by decay through photon emission. By controlling the location of defects in the graphene lattice and the nanostructures around these scattering sites, emission could be localized to regions where this coupling is more optimized. In this second case, the devices may be able to be engineered through nanostructuring in order to gain control over the character of light emission. Finally, a few novel and emerging applications of nanoscale devices will be discussed, including using molecular devices in audio circuits, as well as for high frequency harmonic generation.

DNA Nanotechnology for Cancer Diagnosis and Therapy.

Cancer is one of the leading causes of mortality worldwide, because of the lack of accurate diagnostic tools for the early stages of cancer. Thus, early diagnosis, which provides important information for a timely therapy of cancer, is of great significance for controlling the development of the disease and the proliferation of cancer cells and for improving the survival rates of patients. To achieve the goals of early diagnosis and timely therapy of cancer, DNA nanotechnology may be effective, since it has emerged as a valid technique for the fabrication of various nanoscale structures and devices. The resultant DNA-based nanoscale structures and devices show extraordinary performance in cancer diagnosis, owing to their predictable secondary structures, small sizes, and high biocompatibility and programmability. In particular, the rapid development of DNA nanotechnologies, such as molecular assembly technologies, endows DNA-based nanomaterials with more functionalization and intellectualization. Here, we summarize recent progress made in the development of DNA nanotechnology for the fabrication of functional and intelligent nanomaterials and highlight the prospects of this technology in cancer diagnosis and therapy.

Graphene-mediated self-assembly of gold nanorods into long fibers with controllable optical properties

The paper presents graphene-templated self-assembly of gold nanorods into long fibers with controllable optical properties. It is shown that the size of self-assembled nanostructures varies with time and is controlled by chemical reactions between amine groups of graphene and the functional groups of gold. The absorption peak of the nanostructures changes over time while the photoluminescence intensity is quenched. The self-assembled nanostructures with variable plasmon resonance effects are potentially usable for the fabrication of nanoscale devices for biomedical applications.

Synthesis and Analysis of TiS3 Nanoribbons

Nowadays ,there are a variety of nanoscale materials that scientists around the whole world try to find out specific nanoscale materials which they can truly apply to the industry and the academic.community. Therefore, finding cheap, non-toxic and accessible materials which can actually apply to the industry is being my core target. One-dimensional materials including nanowires, nanobelts, nanoribbons, nanotubes and nanorods that have been attracting a great research in the last few years. These materials have been demonstrated to exhibit superior electrical, optical, mechanical and thermal properties. One-dimensional materials also become the focus of intensive research due to their unique applications in mesoscopic physics and fabrication of nanoscale devices. One-dimensional nanostructures provide convenient system to investigate the dependence of electrical and thermal transport or mechanical properties on dimensionality and size reduction. Furthermore, they are also play an important role as both interconnects and functional units in fabricating electronic, optoelectronic, electrochemical and electromechanical devices with nanoscale dimensions. Metal sulfides exhibit very interesting properties in the fields of energy conversion and storage. Concerning energy conversion, many of them (FeS2, TiS3, MoS2…) show suitable band gap energy and optical properties to be used in photovoltaic systems and photo-electrochemical hydrogen generation. Furthermore, they exhibit 2D-layered structures with the possibility to develop peculiar nano-morphologies such as nanotubes, nanoplates and nanoribbons These materials can be used as nanotransistors because of their optoelectronic properties as well as potential hydrogen stores. Among all metal sulfides, TiS3 is the transition metal trichal-cogenides and quasi-one dimensional semiconductor with outstanding electronic, optoelectronic properties and easy to get. The direct optical band gap of ≈1 eV, high sensitivity and ultrahigh optical responsivities (≈3000 A/W) make them an ideal material for optoelectronic applications. Moreover, TiS3 has large surface area owing to its considerable (001) plane that TiS3 can be applied to the energy storage or hydrogen storage1~2. In this work, titanium based sulfides TiS3 one-dimensional nanostructures (nanoribbons) have been synthesized by chemical vapor deposition (CVD) method in the two-zone heating furnace. Ti and S powder as the precursors of CVD method, which were put into quartz tube and sealed by spray gun. First of all, Ti and S powders were placed between first zone furnace and second zone furnace. Second, formation of sulfides were produced by heating the quartz tube up to 700 ºC. for 40 minutes and held 20 minutes at setting temperature in vacuum. The quartz tube was cooled to 580 ºC for 2 days. The morphology, component, grain size, thickness, length and crystal structure of Ti-sulfides were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and raman spectrum respectively. According to the analyses, X-ray diffraction (XRD) patterns of titanium sulfureted of TiS3 sample at room temperature shows the strongest peak of preferential orientation on the (001) plane but still exhibits strong peak of S of preferential orientation on the (222) plane and strong peak of TiS2 of preferential orientation on the (001) plane. Beyond expectation, there are a lot of TiS2 exhibited which will be the significant influence of the purity of TiS3 nanoribbons. According to literatures, this accident occurs since the decomposition of TiS3 nearly below the eutectic point. The scanning electron microscopy (SEM) images of TiS3 samples with 100 μm in length, 0.5~2 μm in width and 100~200 nm in thickness are observed for TiS3. Furthermore, energy dispersive X-ray spectrometry (EDS) for the synthetized sample that the composition is uniform in agreement with the formation of single phase. The S/Ti ratio of TiS3 ratio is 2.96. Finally , the raman spectrum of every peak of TiS3 sample completely corresponds to the correct position. After doing these analyses, TiS3 nanoribbons are doing analyses of optical and electrical properties and apply to the lithium batteries. Reference: G. Gorlova, V.Ya. Pokrovskii, S.G. Zybtsev, A.N. Titov and V.N. Timofeev. Features of the Conductivity of the Quasi One Dimensional Compound TiS3 Journal of Experimental and Theoretical Physics, 2010, 111, 298–303. Tao, Y.; Wu, X.; Zhang, Y.; Dong, L.; Zhu, J.; Hu, Z. (2008). "Surface-assisted synthesis of microscale hexagonal plates and flower-like patterns of single-crystalline titanium disulfide and their field-emission properties". Crystal Growth and Design. 2990–2994

A chemiresistive sensor array from conductive polymer nanowires fabricated by nanoscale soft lithography.

One-dimensional organic nanostructures are essential building blocks for high performance gas sensors. Constructing an e-nose type sensor array is the current golden standard in developing portable systems for the detection of gas mixtures. However, facile fabrication of nanoscale sensor arrays is still challenging due to the high cost of the conventional nanofabrication techniques. In this work, we fabricate a chemiresistive gas sensor array composed of well-ordered sub-100 nm wide conducting polymer nanowires using cost-effective nanoscale soft lithography. Poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) nanowires functionalized with different self-assembled monolayers (SAMs) are capable of identifying volatile organic compounds (VOCs) at a low concentration range. The side chains and functional groups of the SAMs introduce different sensitivities and selectivities to the targeted analytes. The distinct response pattern of each chemical is subjected to pattern recognition protocols, which leads to a clear separation towards ten VOCs, including ketones, alcohols, alkanes, aromatics and amines. These results of the chemiresistive gas sensor array demonstrate that nanoscale soft lithography is a reliable approach for fabricating nanoscale devices and has the potential of mass producibility.

Lithographic performance of ZEP520A and mr-PosEBR resists exposed by electron beam and extreme ultraviolet lithography

Pattern transfer by deep anisotropic etch is a well-established technique for fabrication of nanoscale devices and structures. For this technique to be effective, the resist material plays a key role and must have a high resolution, reasonable sensitivity, and high etch selectivity against the conventional silicon substrate or underlayer film. In this work, the lithographic performance of two high etch resistance materials was evaluated: ZEP520A (Nippon Zeon Co.) and mr-PosEBR (micro resist technology GmbH). Both materials are positive tone, polymer-based, and nonchemically amplified resists. Two exposure techniques were used: electron beam lithography (EBL) and extreme ultraviolet (EUV) lithography. These resists were originally designed for EBL patterning, where high quality patterning at sub-100 nm resolution was previously demonstrated. In the scope of this work, the authors also aim to validate their extendibility to EUV for high resolution and large area patterning. For this purpose, the same EBL process conditions were employed at EUV. The figures of merit, i.e., dose to clear, dose to size, and resolution, were obtained, and these results are discussed systematically. It was found that both materials are very fast at EUV (dose to clear lower than 12 mJ/cm2) and are capable of resolving dense lines/space arrays with a resolution of 25 nm half-pitch. The quality of patterns was also very good, and the sidewall roughness was below 6 nm. Interestingly, the general-purpose process used for EBL can be extended straightforwardly to EUV lithography with comparably high quality and yield. Our findings open new possibilities for lithographers who wish to devise novel fabrication schemes exploiting EUV for fabrication of nanostructures by deep etch pattern transfer.

Recent Advances in Two-Dimensional Materials with Charge Density Waves: Synthesis, Characterization and Applications

Recently, two-dimensional (2D) charge density wave (CDW) materials have attracted extensive interest due to potential applications as high performance functional nanomaterials. As other 2D materials, 2D CDW materials are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into layers of single unit cell thickness. Although bulk CDW materials have been studied for decades, recent developments in nanoscale characterization and device fabrication have opened up new opportunities allowing applications such as oscillators, electrodes in supercapacitors, energy storage and conversion, sensors and spinelectronic devices. In this review, we first outline the synthesis techniques of 2D CDW materials including mechanical exfoliation, liquid exfoliation, chemical vapor transport (CVT), chemical vapor deposition (CVD), molecular beam epitaxy (MBE) and electrochemical exfoliation. Then, the characterization procedure of the 2D CDW materials such as temperature-dependent Raman spectroscopy, temperature-dependent resistivity, magnetic susceptibility and scanning tunneling microscopy (STM) are reviewed. Finally, applications of 2D CDW materials are reviewed.

Surface morphology evolution during plasma etching of silicon: roughening, smoothing and ripple formation

Atomic- or nanometer-scale roughness on feature surfaces has become an important issue to be resolved in the fabrication of nanoscale devices in industry. Moreover, in some cases, smoothing of initially rough surfaces is required for planarization of film surfaces, and controlled surface roughening is required for maskless fabrication of organized nanostructures on surfaces. An understanding, under what conditions plasma etching results in surface roughening and/or smoothing and what are the mechanisms concerned, is of great technological as well as fundamental interest. In this article, we review recent developments in the experimental and numerical study of the formation and evolution of surface roughness (or surface morphology evolution such as roughening, smoothing, and ripple formation) during plasma etching of Si, with emphasis being placed on a deeper understanding of the mechanisms or plasma–surface interactions that are responsible for. Starting with an overview of the experimental and theoretical/numerical aspects concerned, selected relevant mechanisms are illustrated and discussed primarily on the basis of systematic/mechanistic studies of Si etching in Cl-based plasmas, including noise (or stochastic roughening), geometrical shadowing, surface reemission of etchants, micromasking by etch inhibitors, and ion scattering/chanelling. A comparison of experiments (etching and plasma diagnostics) and numerical simulations (Monte Carlo and classical molecular dynamics) indicates a crucial role of the ion scattering or reflection from microscopically roughened feature surfaces on incidence in the evolution of surface roughness (and ripples) during plasma etching; in effect, the smoothing/non-roughening condition is characterized by reduced effects of the ion reflection, and the roughening-smoothing transition results from reduced ion reflections caused by a change in the predominant ion flux due to that in plasma conditions. Smoothing of initially rough surfaces as well as non-roughening of initially planar surfaces during etching (normal ion incidence) and formation of surface ripples by plasma etching (off-normal ion incidence) are also presented and discussed in this context.