Yuan Liu

University of California, Los Angeles | UCLA

Over the past 70 years, the semiconductor industry has undergone transformative changes, largely driven by the miniaturization of devices and the integration of innovative structures and materials. Two-dimensional (2D) materials like transition metal dichalcogenides (TMDs) and graphene are pivotal in overcoming the limitations of silicon-based tech...

Two-dimensional (2D) semiconductors have shown great potential for monolithic three-dimensional (M3D) integration due to their dangling-bonds-free surface and the ability to integrate to various substrates without the conventional constraint of lattice matching1–10. However, with atomically thin body thickness, 2D semiconductors are not compatible...

The fabrication of perovskite single crystal‐based optoelectronics with improved performance is largely hindered by limited processing techniques. Particularly, the local halide composition manipulation, which dominates the bandgap and thus the formation of heterostructures and emission of multiple‐wavelength light, is realized via prevalent liquid...

Van der Waals (vdW) gaps with ångström-scale heights can confine molecules or ions to an ultimately small scale, providing an alternative way to tune material properties and explore microscopic phenomena. Modulation of the height of vdW gaps between two-dimensional (2D) materials is challenging due to the vdW interaction. Here we report a general a...

Two-dimensional (2D) semiconductors hold great promises for ultra-scaled transistors. In particular, the gate length of MoS2 transistor has been scaled to 1 nm and 0.3 nm using single wall carbon nanotube and graphene, respectively. However, simultaneously scaling the channel length of these short-gate transistor is still challenging, and could be...

Two-dimensional (2D) semiconductors can potentially be used to create scaled electronic devices. However, for a number of promising 2D materials—such as black phosphorus and germanium arsenide—the fabrication of monolayer transistors is challenging and is limited by the difficulties in forming robust electrical contacts with the delicate 2D materia...

Heterostructures made using two-dimensional semiconducting transition metal dichalcogenides could be used to build next-generation electronic devices. However, their performance is limited by low-quality metal–semiconductor contacts, and it remains challenging to create contacts with variable work functions using metals or metallic transition metal...

Vertical transistors, with the channel material sandwiched between graphene and metal electrodes, are promising for the development of next-generation electronic devices. However, realizing complementary transport in two-dimensional vertical transistors is challenging due to the significant disorder and severe Fermi-level pinning effects at the met...

Vertical transistors hold promise for the development of ultrascaled transistors. However, their on/off ratios are limited by a strong source-drain tunneling current in the off state, particularly for vertical devices with a sub-5 nm channel length. Here, we report an approach for suppressing the off-state tunneling current by designing the barrier...

Vertical field effect transistors (VFETs) have attracted considerable interest for developing ultra‐scaled devices. In particular, individual VFET can be stacked on top of another and does not consume additional chip footprint beyond what is needed for a single device at the bottom, representing another dimension for high‐density transistors. Howev...

Strain engineering has been proposed as a promising method to boost the carrier mobility of two-dimensional (2D) semiconductors. However, state-of-the-art straining approaches are largely based on putting 2D semiconductors on flexible substrates or rough substrate with nanostructures (e.g., nanoparticles, nanorods, ripples), where the observed mobi...

Non‐destructive and reversible modulations of polarity and carrier concentration in transistors are essential for complementary devices. The fabricated multi‐gated WSe2 devices obtain dynamic electrostatic field induced electrically configurable functions and demonstrate as diode with high rectification ratio of 4.1 × 10⁵, as well as n‐ and p‐type...

Perovskite heterojunctions are essential components of perovskite optoelectronics, but their construction and investigation have been impeded by the instability and severe anion interdiffusion. Here we epitaxially deposited p-type CsPbBr3 on n-type Nb: SrTiO3 (STO) to construct a functional perovskite heterojunction with high stability. The lattice...

The practical application of two-dimensional (2D) semiconductors for high-performance electronics requires the integration with large-scale and high-quality dielectrics—which however have been challenging to deposit to date, owing to their dangling-bonds-free surface. Here, we report a dry dielectric integration strategy that enables the transfer o...

Two-dimensional semiconductors (2DSCs) have attracted considerable interests for optoelectronic devices, but are often plagued by the difficulties in tailoring the charge doping type and poor optical absorption due to their atomically thin geometry. Herein, we report a methylammonium lead iodide perovskite (CH 3 NH 3 PbI 3 )/2DSC heterojunction dev...

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest for transistor applications. However, the fabrication of 2D transistors using traditional lithography or deposition processes often causes undesired damage and contamination to the atomically thin lattices, partially degrading the device perfo...

Van der Waals (vdW) metallic contacts have been demonstrated as a promising approach to reduce the contact resistance and minimize the Fermi level pinning at the interface of two-dimensional (2D) semiconductors. However, only a limited number of metals can be mechanically peeled and laminated to fabricate vdW contacts, and the required manual trans...

The detection of ultraviolet (UV) radiation with effective performance and robust stability is essential to practical applications. Metal halide single‐crystal perovskites (ABX3) are promising next‐generation materials for UV detection. The device performance of all‐inorganic CsPbCl3 photodetectors (PDs) is still limited by inner imperfection of cr...

Two-dimensional molybdenum disulfide (MoS2) is a potential alternative channel material to silicon for future scaled transistors. Scaling down the gate dielectric and maintaining a high-quality interface is challenging with such materials, because the atomic thickness of MoS2 makes it sensitive to defects common in amorphous gate oxides such as haf...

Logic-in-memory architectures could be used to develop efficient computing devices with low power consumption. However, the approach is limited by device performance issues, including reliability and versatility. Here we report a two-dimensional van der Waals heterostructure device that can function as both reconfigurable transistor and reconfigura...

Two-dimensional (2D) materials have demonstrated promising potential for flexible electronics, owning to their atomic thin body thickness and dangling-bond-free surface. Here, we report a sliding contact device structure for efficient strain releasing. By fabricating a weakly coupled metal-2D junction with a van der Waals (vdW) gap in between, the...

Two-dimensional semiconductors such as layered transition metal dichalcogenides can offer superior immunity to short-channel effects compared with bulk semiconductors such as silicon. As a result, these materials can be used to create highly scaled transistors. However, on-state current densities of two-dimensional semiconductor transistors are sti...

The increase of gate leakage current when the gate dielectric layer is thinned is a key issue for device scalability. For scaling down the integrated circuits, a thin gate dielectric layer with a low leakage current is essential. Currently, changing the dielectric layer material or enhancing the surface contact between the gate dielectric and the c...

Vertical tunneling transistors - in which the channel length is determined by the thickness of the semiconductor - are of interest in the development of next-generation high-density and low-power electronic devices. However, owing to nonideal metal semiconductor interfaces, the majority tunneling carrier type and device polarity are largely fixed,...

Two‐dimensional (2D) materials have attracted extensive attention due to their important prospects in electronics and optoelectronics. Synthesizing new 2D materials, characterizing their properties, and developing their applications are still important topics. Herein, the synthesis of α‐GeTe nanoplates on different substrates via the chemical vapor...

Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene’s plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectr...

Atomically thin monolayer semiconducting transition metal dichalcogenides (TMDs), exhibiting direct band gap and strong light‐matter interaction, are promising for optoelectronic devices. However, an efficient band alignment engineering method is required to further broaden their practical applications as versatile optoelectronics. In this work, th...

Two-dimensional (2D) materials with intrinsic magnetism have drawn intense interest for fundamental research and potential application in spintronics and valleytronics. Here we synthesized 2D Cr3Te4 nanosheets with controllable thickness by chemical vapor deposition approach. Reflection magnetic circular dichroism and magneto-transport measurements...

Front cover image: Ruthenium‐based electrocatalysts have great potential as an alternative to platinum‐based materials for the electrolysis of water for hydrogen evolution. In this work, a unique two‐site mechanism of the ruthenium single/dual atoms and Ru nanoclusters to strengthen the stability of single atom sites and enhance its intrinsic activ...

Schottky diode is the fundamental building blocks for modern electronics and optoelectronics. Reducing the semiconductor layer thickness could shrink the vertical size of a Schottky diode, improving its speed and integration density. Here, we demonstrate a new approach to fabricate a Schottky diode with ultrashort physical length approaching atomic...

Two-dimensional (2D) semiconductors show great potential for high-performance electronic devices. With atomically thin 2D lattice, achieving high quality and reliable metal contact remains a critical challenge. van der Waals (vdW) contact geometry — in which metal electrodes are physically laminated onto 2D surface — has demonstrated reduced contac...

2D Semiconductors are promising in the development of next‐generation photodetectors. However, the performances of 2D photodetectors are largely limited by their poor light absorption (due to ultrathin thickness) and small detection range (due to large bandgap). To overcome the limitations, a strain‐plasmonic coupled 2D photodetector is designed by...

Artificially stacked van der Waals heterostructures (vdWH) of two-dimensional (2D) atomic layers have attracted considerable attention due to substantial interactions between different layers. In particular, the strongly bound interlayer exciton (IX) within vdWH offers a platform for exploring fundamental physics as well as innovative device applic...

The fundamental Boltzmann limitation dictates the ultimate limit of subthreshold swing (SS) to be 60 mV dec⁻¹, which prevents the continued scaling of supply voltage. With atomically thin body, 2D semiconductors provide new possibilities for advanced low-power electronics. Herein, ultra-steep-slope MoS2 resistive-gate field-effect transistors (RG-F...

In recent years, research into the synthesis and applications of 0D carbon dots (CDs) has blossomed into a vibrant and exciting new research field. CDs possess diverse and fascinating chemical, structural, and optical characteristics, which can be exploited in both fundamental research and applied areas. In particular, their superior electrochemica...

Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene’s plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectr...

Memristors have attracted tremendous interest in the fields of high-density memory and neuromorphic computing. However, despite the tremendous efforts that have been devoted over recent years, high operating voltage, poor stability, and large device variability remain key limitations for its practical application and can be partially attributed to...

Currently, the most efficient electrocatalyst for the hydrogen evolution reaction (HER) in water dissociation is Pt‐based catalyst. Unfortunately, the high cost and less than perfect efficiency hinder wide‐range industrial/technological applications. Here, by controlling the treatment temperature of tris (2,2‐bipyridine) ruthenium dichloride hexahy...

2D semiconductors present tunable property with the physical dimension. Herein, an efficient strategy to modulate the band structure of ultrathin channel by dimension tailoring of the 2D materials is reported. In order to verify the practicability of this strategy, bulk‐MoS2/MoS2 nanoribbon (NR) homojunctions are constructed with a rectification ra...

The two-dimensional transition metal dichalcogenides (TMDs) have attracted intense interest as an atomically thin semiconductor channel for the continued transistor scaling. However, with a dangling bond free surface, it has been a key challenge to reliably integrate high-quality gate dielectrics on TMDs. In particular, the atomic layer deposition...

In this paper, negative transconductance (NTC) behavior in molybdenum disulfides (MoS2) field effect transistors (FETs) is investigated. Combining experimental observation and numerical analysis, we demonstrate that positive shift in the device transfer curves results from the electron trapping/de-trapping processes, where the defect densities at t...

The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe2 nanosheets can be readily grown on a dangling-bond-free WSe2 substrate with systematically tunable thickness down to...

2D semiconductors have attracted tremendous attention as an atomically thin channel for transistors with superior immunity to short‐channel effects. However, with atomic thin structure, the delicate 2D lattice is not fully compatible with conventional lithography processes that typically involve high‐energy photon/electron radiation and unavoidable...

Numerous efforts have been made to synthesis two-dimensional (2D) atomic semiconductor materials and their heterojunctions owing to diverse novel properties and potential applications in constructing next generation highly compact electronics and optoelectronics devices. However, the intrinsic 2D p-type semiconductor material is still scarce to dat...

Vertical transistors—in which the channel length is determined by the thickness of the semiconductor—are of interest in the development of next-generation electronic devices. However, short-channel vertical devices are difficult to fabricate, because the high-energy metallization process typically results in damage to the contact region. Here we sh...

Cost-effective production of metal-free catalysts for the oxygen-reduction reaction (ORR), to supersede Pt-based catalysts, is challenging. Here, a three-dimensional nanocatalyst was prepared by compounding multi-wall carbon nanotubes (MWCNTs) with easily modified and doped carbon dots (CDs) as sources of B and N. The catalyst has high conductivity...

Van der Waals heterostructures (vdWHs) have attracted tremendous interest owing to the ability to assemble diverse building blocks without the constraints of lattice matching and processing compatibility. However, once assembled, the fabricated vdWHs can hardly be separated into individual building blocks for further manipulation, mainly due to tec...

Two-dimensional (2D) materials1,2 and the associated van der Waals (vdW) heterostructures3–7 have provided great flexibility for integrating distinct atomic layers beyond the traditional limits of lattice-matching requirements, through layer-by-layer mechanical restacking or sequential synthesis. However, the 2D vdW heterostructures explored so far...

The electronic fluctuation issue of the armchair graphene nanoribbons (AGNRs) is theoretically investigated under a full quantum dynamics framework. Besides the electrons, the behaviors of the C and H nuclei are also quantized, more rigorous than the common classical particle treatment. Simulation results show that the nuclear quantization will boo...

Two-dimensional (2D) semiconductors have attracted tremendous interest as atomically thin channels that could facilitate continued transistor scaling. However, despite many proof-of-concept demonstrations, the full potential of 2D transistors has yet to be determined. To this end, the fundamental merits and technological limits of 2D transistors ne...

The synthesis of 2D WSe2 nanosheets, CoSe‐WSe2 lateral heterostructures. The field‐effect transistors (FETs) of CoSe‐WSe2 lateral heterostructures show satisfactory Ohmic contacts and considerably better FET performance over those with deposited Cr/Au contacts, suggesting the in‐plane metal‐semiconductor junctions may function as improved contacts...

Designing bifunctional catalysts capable of driving the electrochemical hydrogen evolution reaction (HER) and also H2 evolution via the hydrolysis of hydrogen storage materials such as ammonia borane (AB) is of considerable practical importance for future hydrogen economies. Herein, we systematically examined the effect of tensile lattice strain in...

Designing bifunctional catalysts capable of driving the electrochemical hydrogen evolution reaction (HER) and also H2 evolution via the hydrolysis of hydrogen storage materials such as ammonia borane (AB) is of considerable practical importance for future hydrogen economies. Herein, we systematically examined the effect of tensile lattice strain in...

[This corrects the article DOI: 10.1093/nsr/nwz223.].

Two-dimensional (2D) semiconductors are attractive for electronic devices with atomically thin channels. However, controlling the electronic properties of the 2D materials by incorporating impurity dopants is inherently difficult due to the limited physical space in the atomically thin lattices. Here we show that a solid-state ionic doping approach...

The two‐dimensional (2D) in‐plane (lateral) heterostructures have attracted increasing interest for potential applications in the atomically thin electronics and optoelectronics. While most studies focus on semiconductor‐semiconductor lateral heterostructures with highly similar lattice structures between the constituent components, the synthesis o...

Lead halide perovskites have attracted increasing interest for their exciting potential in diverse optoelectronic devices. However, their charge transport properties remain elusive, plagued by the issues of excessive contact resistance and large hysteresis in ambient conditions. Here we report a van der Waals integration approach for creating high-...

With the continued scaling of transistors, there is a growing trend for developing steep slope transistors with subthreshold swing (SS) below Boltzmann limitation (kT/q). To this end, impact ionization metal oxide semiconductor (I‐MOS) transistors are attractive for a unique combination of high ON‐state current density, small hysteresis, and ultra‐...

Two-dimensional (2D) Ruddlesden-Popper perovskites are currently drawing significant attention as highly-stable photoactive materials for optoelectronic applications. However, the insulating nature of organic ammonium layers in 2D perovskites results in poor charge transport and limited performance. Here, we demonstrate that Al2O3/2D perovskite het...

Metal-oxide-semiconductor field effect transistors (MOSFET) based on two-dimensional (2D) semiconductors have attracted extensive attention owing to their excellent transport properties, atomically thin geometry, and tunable bandgaps. Besides improving the transistor performance of individual device, lots of efforts have been devoted to achieving 2...

The rapid development of mobile and internet‐of‐thing devices demands continuous scaling of metal–insulator–semiconductor field‐effect transistors for high‐resolution and low‐power displays. However, such technology is limited by inadequate scaling of supply voltage and sophisticated dielectric engineering. Here, to enable continued scaling, indium...

Molybdenum disulfide (MoS2) holds great promise as atomically thin two-dimensional (2D) semiconductor for future electronics and opto-electronics. In this report, we study the magnetoresistance (MR) of MoS2 field-effect transistors (FETs) with graphene insertion layer at the contact interface. Owing to the unique device structure and high-quality c...

Palladium diselenide (PdSe2), a stable layered material with pentagonal structure, has attracted extensive interest due to its excellent electrical and optoelectronic performance. Here, we report a reliable process to synthesize PdSe2 via chemical vapor deposition (CVD) method. Through systematic regulation of temperature in the growth process, we...

Due to the backscattered parasitic current from the barriers, the current gain of the widely used amplifier is far from ideal. In this work, we demonstrate a vertical Au/Al2O3/BP/MoS2 tunneling hot-electron transfer amplifier with a hot-electron emitter-base junction and a p-n junction as the base-collector barrier. Fairly monoenergetic electrons t...

Two-dimensional (2D) semiconductors have attracted considerable attention for the development of ultra-thin body transistors. However, the polarity control of 2D transistors and the achievement of complementary logic functions remain critical challenges. Here, we report a doping-free strategy to modulate the polarity of WSe2 transistors using same...

Monolayer transition metal dichalcogenides (TMDs) have attracted considerable attention as atomically thin semiconductors for the ultimate transistor scaling. For practical applications in integrated electronics, large monolayer single crystals are essential for ensuring consistent electronic properties and high device yield. The TMDs available tod...

Strain engineering is a promising method to manipulate the electronic and optical properties of two-dimensional (2D) materials. However, with weak van der Waals interaction, severe slippage between 2D material and substrate could dominate the bending or stretching processes, leading to inefficiency strain transfer. To overcome this limitation, we r...

Two-dimensional van der Waals heterostructures (vdWHs) have attracted considerable interest1–4. However, most vdWHs reported so far are created by an arduous micromechanical exfoliation and manual restacking process5, which—although versatile for proof-of-concept demonstrations6–16 and fundamental studies17–30—is clearly not scalable for practical...

Toward memristor scaling, it becomes increasingly challenging to maintain reliable switching as the interelectrode distance shrinks to smaller and smaller scale. In particular, the memristive active layers are usually fragile and prone to metal-integration-induced damage. To adapt an ultrathin active switching layer for low-voltage operation requir...

In atomically-thin two-dimensional (2D) semiconductors, the nonuniformity in current flow due to its edge states may alter and even dictate the charge transport properties of the entire device. However, the influence of the edge states on electrical transport in 2D materials has not been sufficiently explored to date. Here, we systematically quanti...

Microscopic pressure sensors that can rapidly detect small pressure variations are of value in robotic technologies, human–machine interfaces, artificial intelligence and health monitoring devices. However, both capacitive and transistor-based pressure sensors have limitations in terms of sensitivity, response speed, stability and power consumption...