Jiuxiang Dai

Nature, Published online: 11 May 2022; doi:10.1038/s41586-022-04588-2

Huge achievements have been made in graphene chemical vapor deposition (CVD) growth. This review systematical summarizes the current progresses in four research directions, including theoretical study of graphene CVD growth, direct growth on insulating substrates, low temperature growth, layer number, and stacking-angle controlled growth. Finally, the future research directions of graphene are discussed.

Abstract

Graphene, since the first successful exfoliation of graphite, has continuously attracted attention due to its remarkable properties and applications. Recently, the research focus on graphene synthesis has been directed to the controllable synthesis of large-area and high-quality graphene. In the last decade, there has been great progress in the chemical vapor deposition (CVD) growth of graphene. Theoretical investigations have led to an enhanced understanding of puzzles on hydrocarbon species stability, key reaction pathways, the role of hydrogen gas, the morphology of graphene islands, and the alignment of graphene on substrates. Experimentally, high-quality graphene is epitaxially grown on both insulating and metal substrates. Progress has also been reported on low-temperature graphene growth and on controlling the thickness and stacking of graphene layers. In this review, the authors summarize the previous theoretical and experimental studies on graphene CVD growth and discuss the future challenges on the growth of graphene i) on insulating substrates, ii) at low temperature, iii) with controllable thickness, and iv) with selected stacking twist angles. The authors assert that the key to the continuous advancement of graphene growth is the synergy of experimental and theoretical investigations.

Recent advances in emerging 2D-material-based integrated memory devices are reviewed in terms of working principles, device architectures, array integration, and specific brain-inspired applications. Future challenges and promising research lines toward reliable, practical neuromorphic computing chips are highlighted.

Abstract

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Author(s): Kaveh Khaliji, Luis Martín-Moreno, Phaedon Avouris, Sang-Hyun Oh, and Tony Low

The ability to control the light polarization state is critically important for diverse applications in information processing, telecommunications, and spectroscopy. Here, we propose that a stack of anisotropic van der Waals materials can facilitate the building of optical elements with Jones matric…

[Phys. Rev. Lett. 128, 193902] Published Thu May 12, 2022

2D Bi₂O₂Se single crystals are synthesized efficiently by organic ion template-guided solution method using ammonium bismuth citrate and are transformed into ultrathin nanosheets after being washed. High-performance Bi₂O₂Se optoelectronic devices are fabricated by using transfer-free Bi₂O₂Se nanosheets on SiO₂/Si substrate.

Abstract

2D Bi₂O₂Se has recently attracted widespread research interest due to its high electron mobility and good air stability with tunable bandgap. However, the direct and controllable growth of large-size, high-quality ultrathin Bi₂O₂Se with tunable electronic properties remains a great challenge. Here, an organic ion template-guided solution growth method, is developed using water as a solvent to efficiently obtain high-quality 2D Bi₂O₂Se. Significantly, the thicknesses and morphologies of 2D Bi₂O _x Se with various oxygen deficiencies; are also achieved. Optical spectroscopic results indicate that oxygen defects can dramatically tune the electronic properties of Bi₂O₂Se, such as work function, band gap, and energy-band positions. As a result, the optimized Bi₂O₂Se devices exhibit a high photoresponsivity of 842.91 A W⁻¹, photo-detectivity of 8.18 × 10¹² Jones under 532 nm laser, and electron mobility of 334.7 cm² V⁻¹ s⁻¹. This work proposes a new green solution method to synthesize large-sized, high-quality Bi₂O₂Se with tunable electronic structures that can be extended for various applications and provides a new possibility of transfer-free large-scale device manufacturing.

This work realizes a thick Bi₂Te₃-based thermoelectric film with a “graphene/Bi₂Te₃/graphene” sandwiched structure, which demonstrates an unprecedentedly high figure of merit for flexibility among all Bi₂Te₃-based films ever reported, due to the outstanding intrinsic flexibility of graphene and a small slippage barrier.

Abstract

Flexible thermoelectrics (TEs) that fit curved human skin well, could harvest energy from skin, and thus have been considered as a promising portable power source for wearable electronics. Bi₂Te₃, the most popular room-temperature TE material, is still challenging to be applied in flexible devices due to its rigid nature. Although many Bi₂Te₃-based films have been reported to be flexible when made thin enough, the thermal and electrical loads across them are rather small with severe limitation on the maximum power output. This work realizes a thick Bi₂Te₃-based TE film with a “graphene/Bi₂Te₃/graphene” sandwiched structure, which demonstrates an unprecedentedly high figure of merit for flexibility among all Bi₂Te₃-based films ever reported, due to the outstanding intrinsic flexibility of graphene and a small slippage barrier. Meanwhile, graphene acts as express conducting channels as well as carrier donors, resulting in an increased electrical conductivity. The numerous graphene/Bi₂Te₃ heterointerfaces induce energy filtering effect, leading to an enhanced Seebeck coefficient, and thus an optimized power factor is achieved. This work offers a cost-effective avenue to make highly flexible TE films for power supply of wearable electronics by intercalating TE nanoplates into 2-dimensional nanosheets.

The photodetectors based on ultrathin Sb₂Se₃ nanowires present high device performance, especially a large signal/noise ratio of 1436.55 and an extremely low shot noise of ≈9 × 10^-16 A Hz^-1/2. More interestingly, the Sb₂Se₃ nanowire photodetectors exhibit broadband polarized photoresponse and appropriate polarimetric imaging quality, revealing the potential of Sb₂Se₃ nanowires for polarimetric imaging applications.

Abstract

This work presents a study on the optical applications of chemical vapor deposition-grown Sb₂Se₃ nanowires in polarized single nanowire photodetectors. High-quality Sb₂Se₃ nanowires are obtained with diameters as small as ≈15 nm, which is the first report for ultrathin Sb₂Se₃ nanowires. The fabricated Sb₂Se₃ nanowire-based photodetector presents a low shot noise of ≈ 9 × 10^–16 A Hz^–1/2, a large signal/noise ratio of 1436.55, a high responsivity of 3.61 A W^–1, and a high specific detectivity of 2.36 × 10¹¹ Jones, which can be attributed to the high-quality crystalline nanowires obtained. More interestingly, the Sb₂Se₃ nanowire-based photodetectors exhibit broadband polarized photoresponse to incident light with wavelengths ranging from visible to near-infrared (532 – 830 nm). A linearly dichroic ratio of 1.71 is obtained for the 830 nm light illumination. The Sb₂Se₃ nanowire detectors also present appropriate polarimetric imaging quality, revealing the potential of Sb₂Se₃ nanowires for polarimetric imaging applications.

A deep ultraviolet (DUV) photodetector based on an ultrathin non-ultrawide bandgap semiconductor GaSe nanobelt is fabricated. The blueshift of peak response with decreasing thickness is ascribed to the wavelength-dependent absorption coefficient of GaSe as well as the enhanced light–matter interaction arising from the longitudinal Fabry–Pérot (F–P) cavity resonance.

Abstract

In this paper, the authors report the fabrication of a sensitive deep ultraviolet (DUV) photodetector by using an individual GaSe nanobelt with a thickness of 52.1 nm, which presents the highest photoresponse at 265 nm illumination with a responsivity and photoconductive gain of about 663 A W⁻¹ and 3103 at a 3 V bias, respectively, comparable to or even better than other reported devices based on conventional wide bandgap semiconductors. According to the simulation, this photoelectric property is associated with the wavelength-dependent absorption coefficient of the GaSe crystal, for which incident light with shorter wavelengths will be absorbed near the surface, while light with longer wavelengths will have a larger penetration depth, leading to a blueshift of the absorption edge with decreasing thickness. Further finite element method (FEM) simulation reveals that the relatively thin GaSe nanobelt exhibits an enhanced transversal standing wave pattern compared to its thicker counterpart at a wavelength of 265 nm, leading to an enhanced light–matter interaction and thereby more efficient photocurrent generation. The device can also function as an effective image sensor with acceptable spatial resolution. This work will shed light on the facile fabrication of a high-performance DUV photodetector from non-ultrawide bandgap semiconductors.

Ge-based devices are promising next-generation technologies for extending Moore's law. However, Fermi-level pinning (FLP) at the metal/n-Ge interface results in large contact resistance. Here, a wafer-scale conductive carbon nanotube insertion method is developed to alleviate FLP, leading to ohmic contact and the smallest contact resistance between a metal and a lightly doped n-Ge.

Abstract

Germanium (Ge)-based devices are recognized as one of the most promising next-generation technologies for extending Moore's law. However, one of the critical issues is Fermi-level pinning (FLP) at the metal/n-Ge interface, and the resulting large contact resistance seriously degrades their performance. The insertion of a thin layer is one main technique for FLP modulation; however, the contact resistance is still limited by the remaining barrier height and the resistance induced by the insertion layer. In addition, the proposed depinning mechanisms are also controversial. Here, the authors report a wafer-scale carbon nanotube (CNT) insertion method to alleviate FLP. The inserted conductive film reduces the effective Schottky barrier height without inducing a large resistance, leading to ohmic contact and the smallest contact resistance between a metal and a lightly doped n-Ge. These devices also indicate that the metal-induced gap states mechanism is responsible for the pinning. Based on the proposed technology, a wafer-scale planar diode array is fabricated at room temperature without using the traditional ion-implantation and annealing technology, achieving an on-to-off current ratio of 4.59 × 10⁴. This work provides a new way of FLP modulation that helps to improve device performance with new materials.

Nature Nanotechnology, Published online: 12 May 2022; doi:10.1038/s41565-022-01124-1

A summary of advances in developing surface-enhanced Raman spectroscopy (SERS)-active substrates via fabrication methods of nanostructured metals and 2D materials is provided. Approaches for controlling metal tips/gaps/pores including advantages and disadvantages are discussed. Moreover, recent advances and challenges of novel 2D-layered SERS-active substrates are presented. Functionalization strategies for SERS surfaces include use of small or large molecules to complement the preparation procedures.

Abstract

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

High quality single-layer MoS₂ flakes are directly grown on the trenched SrTiO₃(111) substrate. Combination of multiple microscopy techniques including tip-enhanced photoluminescence, tunneling atomic force microscopy, and scanning tunneling microscopy reveals that the high steps of SrTiO₃ reduce the bandgap of the MoS₂ adlayer and lead to the photoluminescence quenching of the MoS₂.

Abstract

The structure and morphology of the substrate surface play critical roles in tuning the properties of the supported two-dimension materials (2DM). In this work, a simple strategy to engineer the SrTiO₃ single crystal into a trenched structure which is composed of atomically flat terraces and high steps of several nanometers is developed. Through the conventional chemical vapor deposition method, high quality single-layered MoS₂ nanosheets are successfully fabricated directly on the trenched SrTiO₃ (Tr-STO) substrate, which thus result in a heterostructure with well-defined interface and controllable corrugated morphology. The corrugated MoS₂/Tr-STO sample displays a drastically suppressed photoluminescence as compared to those grown on atomically flat substrates. Detailed scanning probe microscopy in combination with optical spectroscopy measurements demonstrates that the photoluminescence quenching occurs exclusively in the MoS₂ area carpeting the high SrTiO₃ steps, which can be attributed to the significantly reduced bandgaps hence massively enriched free charges in these regions. This work not only provides a new strategy to tailor the 2DM properties by simply engineering the substrate surface corrugations, but also brings deep insights into the dependence of properties of the hybridized system on the interface morphologies.

The authors report on the fabrication and application of a ferroelectric transistor integrated with a van der Waals ferroelectrics heterostructure (CuInP₂S₆/α-In₂Se₃). Leveraging enhanced polarization originating from the dipole coupling, the fabricated device exhibits a large memory window and nonvolatile memory characteristics with long retention time and stable cyclic endurance, providing a promising pathway for exploring low-dimensional ferroelectronics.

Abstract

To address the demands of emerging data-centric computing applications, ferroelectric field-effect transistors (Fe-FETs) are considered the forefront of semiconductor electronics owing to their energy and area efficiency and merged logic–memory functionalities. Herein, the fabrication and application of an Fe-FET, which is integrated with a van der Waals ferroelectrics heterostructure (CuInP₂S₆/α-In₂Se₃), is reported. Leveraging enhanced polarization originating from the dipole coupling of CIPS and α-In₂Se₃, the fabricated Fe-FET exhibits a large memory window of 14.5 V at V _GS = ±10 V, reaching a memory window to sweep range of ≈72%. Piezoelectric force microscopy measurements confirm the enhanced polarization-induced wider hysteresis loop of the double-stacked ferroelectrics compared to single ferroelectric layers. The Landau–Khalatnikov theory is extended to analyze the ferroelectric characteristics of a ferroelectric heterostructure, providing detailed explanations of the hysteresis behaviors and enhanced memory window formation. The fabricated Fe-FET shows nonvolatile memory characteristics, with a high on/off current ratio of over 10⁶, long retention time (>10⁴ s), and stable cyclic endurance (>10⁴ cycles). Furthermore, the applicability of the ferroelectrics heterostructure is investigated for artificial synapses and for hardware neural networks through training and inference simulation. These results provide a promising pathway for exploring low-dimensional ferroelectronics.

D. Yagodkin, K. Greben, A. Eljarrat, S. Kovalchuk, M. Ghorbani-Asl, M. Jain, S. Kretschmer, N. Severin, J. P. Rabe, A. V. Krasheninnikov, C. T. Koch, K. I. Bolotin

A new excitonic state is observed in 2D semiconductors after electron beam functionalization. The state is bright, has a very narrow photoluminescence peak at cryogenic temperatures, and survives up to room temperature. It is shown that this state is not related to intrinsic defects in transition metal dichalcogenides but is associated with molecules on the surface of the material.

A new localized excitonic state is demonstrated in patterned monolayer 2D semiconductors. The signature of an exciton associated with that state is observed in the photoluminescence spectrum after electron beam exposure of several 2D semiconductors. The localized state, which is distinguished by non-linear power dependence, survives up to room temperature and is patternable down to 20 nm resolution. The response of the new exciton to the changes of electron beam energy, nanomechanical cleaning, and encapsulation via multiple microscopic, spectroscopic, and computational techniques is probed. All these approaches suggest that the state does not originate from irradiation-induced structural defects or spatially non-uniform strain, as commonly assumed. Instead, it is shown to be of extrinsic origin, likely a charge transfer exciton associated with the organic substance deposited onto the 2D semiconductor. By demonstrating that structural defects are not required for the formation of localized excitons, this work opens new possibilities for further understanding of localized excitons as well as their use in applications that are sensitive to the presence of defects, e.g. chemical sensing and quantum technologies.