Fundamental band gap and alignment of two-dimensional semiconductors explored by machine learning

Two-dimensional(2D)semiconductors isoelectronic to phosphorene have been drawing much attention recently due to their promising applications for next-generation(opt)electronics.This family of 2D materials contains more than 400members,including(a)elemental group-V materials,(b)binary III–VII and IV–VI compounds,(c)ternary III–VI–VII and IV–V–VII compounds,making materials design with targeted functionality unprecedentedly rich and extremely challenging.To shed light on rational functionality design with this family of materials,we systemically explore their fundamental band gaps and alignments using hybrid density functional theory(DFT)in combination with machine learning.First,calculations are performed using both the Perdew–Burke–Ernzerhof exchange–correlation functional within the generalgradient-density approximation(GGA-PBE)and Heyd–Scuseria–Ernzerhof hybrid functional(HSE)as a reference.We find this family of materials share similar crystalline structures,but possess largely distributed band-gap values ranging approximately from 0 eV to 8 eV.Then,we apply machine learning methods,including linear regression(LR),random forest regression(RFR),and support vector machine regression(SVR),to build models for the prediction of electronic properties.Among these models,SVR is found to have the best performance,yielding the root mean square error(RMSE)less than 0.15 eV for the predicted band gaps,valence-band maximums(VBMs),and conduction-band minimums(CBMs)when both PBE results and elemental information are used as features.Thus,we demonstrate that the machine learning models are universally suitable for screening 2D isoelectronic systems with targeted functionality,and especially valuable for the design of alloys and heterogeneous systems.

A family of flexible two-dimensional semiconductors:MgMX2Y6(M=Ti/Zr/Hf;X=Si/Ge;Y=S/Se/Te)

Inspired by the recently predicted 2D MX_(2)Y_(6)(M=metal element;X=Si/Ge/Sn;Y=S/Se/Te),we explore the possible applications of alkaline earth metal(using magnesium as example)in this family based on the idea of element replacement and valence electron balance.Herein,we report a new family of 2D quaternary compounds,namely MgMX_(2)Y_(6)(M=Ti/Zr/Hf;X=Si/Ge;Y=S/Se/Te)monolayers,with superior kinetic,thermodynamic and mechanical stability.In addition,our results indicate that MgMX_(2)Y_(6)monolayers are all indirect band gap semiconductors with band gap values ranging from 0.870 to 2.500 eV.Moreover,the band edges and optical properties of 2D MgMX_(2)Y_(6)are suitable for constructing multifunctional optoelectronic devices.Furthermore,for comparison,the mechanical,electronic and optical properties of In_(2)X_(2)Y_(6)monolayers have been discussed in detail.The success of introducing Mg into the 2D MX_(2)Y_(6)family indicates that more potential materials,such as Caand Sr-based 2D MX_(2)Y_(6)monolayers,may be discovered in the future.Therefore,this work not only broadens the existing family of 2D semiconductors,but it also provides beneficial results for the future.

Toward intrinsic room-temperature ferromagnetism in two-dimensional semiconductors

Two-dimensional (2D) ferromagnetic semiconductors have been recognized as the most promising candidates for next-generation low-cost, high-performance and nano-scale spintronic applications such as spin field-effect transistors and quantum computation/communication. However, as one of the 125 important scientific issues raised by Science journal in 2005 that "is it possible to create magnetic semiconductors that work at room temperature?", how to achieve a feasible ferromagnetic semiconductor with high Curie temperature is still a long-standing challenge despite of tremendous efforts have been devoted in this field since 1960s. The recent discovery of 2D ferromagnetic semiconductors Cr2Ge2Te6 and CrI3 has evoked new research interests in 2D intrinsic ferromagnetic semiconductors. But the low Curie temperature (<45 K) of these materials is still badly hindering their industrial applications.

Contact engineering for two-dimensional semiconductors

Two-dimensional(2D)layered materials,including graphene,black phosphorus(BP)and transition metal dichalcogenide(TMD)such as molybdenum disulfide(Mo S2),tungsten diselenide(WSe2),have attracted increasing attention for the application in electronic and optoelectronic devices.Contacts,which are the communication links between these 2D materials and external circuitry,have significant effects on the performance of electronic and optoelectronic devices.However,the performance of devices based on 2D semiconductors(SCs)is often limited by the contacts.Here,we provide a comprehensive overview of the basic physics and role of contacts in 2D SCs,elucidating Schottky barrier nature and Fermi level pinning effect at metal/2D SCs contact interface.The progress of contact engineering,including traditional metals contacts and metallic 2D materials contacts,for improving the performance of 2D SCs based devices is presented.Traditional metal contacts,named 3D top and edge contacts,are discussed briefly.Meanwhile,methods of building 2D materials contacts(2D top contact and 2D edge contact)are discussed in detail,such as chemical vapor deposition(CVD)growth of 2D metallic material contacts,phase engineered metallic phase contacts and intercalation induced metallic state contacts.Finally,the challenges and opportunities of contact engineering for 2D SCs are outlined.

Recent progresses of NMOS and CMOS logic functions based on two-dimensional semiconductors

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 2D logic functions or integrated circuit towards practical application.In this review,we discussed the recent progresses of 2D-based logic circuit.We will first start with the different methods for realization of n-type metal-oxide-semiconductor(NMOS)-only(or p-type metal-oxide-semiconductor(PMOS)-only)logic circuit.Next,various device polarity control and complementary-metal-oxide-semiconductor(CMOS)approaches are summarized,including utilizing different 2D semiconductors with intrinsic complementary doping,charge transfer doping,contact engineering,and electrostatics doping.We will discuss the merits and drawbacks of each approach,and lastly conclude with a short perspective on the challenges and future developments of 2D logic circuit.

Two-dimensional Cr_(2)Cl_(3)S_(3) Janus magnetic semiconductor with large magnetic exchange interaction and high-T_(C)

The two-dimensional(2D)Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures.Van der Waals CrCl_(3) monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K,which will limit its application in spintronic devices.In this work,we propose a new Janus monolayer Cr_(2)Cl_(3)S_(3) based on the first principles calculations.The phonon dispersion and elastic constants confirm that Janus monolayer Cr_(2)Cl_(3)S_(3) is dynamically and mechanically stable.Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr_(2)Cl_(3)S_(3) is an intrinsic ferromagnetic semiconductor with TC of 180 K,which is much higher than that of CrCl_(3) due to the enhanced ferromagnetic coupling caused by S substitution.Moreover,the magnetic easy axis of Janus Cr_(2)Cl_(3)S_(3) can be tuned to the perpendicular direction with a large magnetic anisotropy energy(MAE)of 142eV/Cr.Furthermore,the effect of biaxial strain on the magnetic property of Janus monolayer Cr_(2)Cl_(3)S_(3) is evaluated.It is found that the Curie temperature is more robust under tensile strain.This work indicates that the Janus monolayer Cr_(2)Cl_(3)S_(3) presents increased Curie temperature and out-of-plane magnetic easy axis,suggesting greater application potential in 2D spintronic devices.

Approaching ohmic contact to two-dimensional semiconductors

Two-dimensional semiconductors have attracted immense research interests owing to their intriguing properties and promising applications in electronic and optoelectronic devices.However,the performance of these devices is drastically hindered by the large Schottky barrier at the electric contact interface,which is hardly tunable due to the Fermi level pinning effect.In this review,we will analyze the root causes of the contact problems for the two-dimensional semiconductor devices and summarize the strategies on the basis of different contact geometries,aiming to lift out the Fermi level pinning effect and achieve the ohmic contact.Moreover,the remarkable improvement of the device performance thanks to these optimized contacts will be emphasized.At the end,the merits and limitations of these strategies will be discussed as well,which potentially gives a guideline for handling the electric contact issues in two-dimensional semiconductors devices.

Top gate engineering of field-effect transistors based on wafer-scale two-dimensional semiconductors

The investigation of two-dimensional(2D)materials has advanced into practical device applications,such as cascaded logic stages.However,incompatible electrical properties and inappropriate logic levels remain enormous challenges.In this work,a doping-free strategy is investigated by top gated(TG)MoS_(2) field-effect transistors(FETs)using various metal gates(Au,Cu,Ag,and Al).These metals with different work functions provide a convenient tuning knob for controlling threshold voltage(V_(th))for MoS_(2) FETs.For instance,the Al electrode can create an extra electron doping(n-doping)behavior in the MoS_(2) TG-FETs due to a dipole effect at the gate-dielectric interface.In this work,by achieving matched electrical properties for the load transistor and the driver transistor in an inverter circuit,we successfully demonstrate wafer-scale MoS_(2) inverter arrays with an optimized inverter switching threshold voltage(V_(M))of 1.5 V and a DC voltage gain of 27 at a supply voltage(V_(DD))of 3 V.This work offers a novel scheme for the fabrication of fully integrated multistage logic circuits based on wafer-scale MoS_(2) film.

High temperature magnetic semiconductors: narrow band gaps and two-dimensional systems

Magnetic semiconductors have been demonstrated to work at low temperatures, but not yet at room temperature for spin electronic applications. In contrast to the p-type diluted magnetic semiconductors, n-type diluted magnetic semiconductors are few. Using a combined method of the density function theory and quantum Monte Carlo simulation, we briefly discuss the recent progress to obtain diluted magnetic semiconductors with both p- and n-type carriers by choosing host semiconductors with a narrow band gap. In addition, the recent progress on two-dimensional intrinsic magnetic semiconductors with possible room temperature ferromangetism and quantum anomalous Hall effect are also discussed.

Two-dimensional semiconductors with possible high room temperature mobility

We have calculated the longitudinal acoustic phonon limited electron mobility of 14 twvo-dimensional semiconductors with composition of MX2, where M （= Mo, W, Sn, Hf, Zr and Pt） is the transition metal, and X is S, Se and Te. We treated the scattering matrix by the deformation potential approximation. We found that out of 14 compounds, MoTe2, HfSe2 and ZrSe2 are promising regarding to their possible high mobility and finite band gap. The phonon limited mobility can be above 2,500 cm^2·V^-1·s^-1 at room temperature.

Phonon-assisted upconversion in twisted two-dimensional semiconductors

Phonon-assisted photon upconversion(UPC)is an anti-Stokes process in which incident photons achieve higher energy emission by absorbing phonons.This letter studies phonon-assisted UPC in twisted 2D semiconductors,in which an inverted contrast between UPC and conventional photoluminescence(PL)of WSe2 twisted bilayer is emergent.A 4-fold UPC enhancement is achieved in 5.5°twisted bilayer while PL weakens by half.Reduced interlayer exciton conversion efficiency driven by lattice relaxation,along with enhanced pump efficiency resulting from spectral redshift,lead to the rotation-angle-dependent UPC enhancement.The counterintuitive phenomenon provides a novel insight into a unique way that twisted angle affects UPC and light-matter interactions in 2D semiconductors.Furthermore,the UPC enhancement platform with various superimposable means offers an effective method for lighting bilayers and expanding the application prospect of 2D stacked van der Waals devices.

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional （2D） layered materials have potential applications in nanoelectronics, nanophoton- ics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes （LEDs）, and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.

Co_(6)Se_(8)(PEt_(3))_(6) superatoms as tunable chemical dopants for two-dimensional semiconductors

Electronic,optoelectronic,and other functionalities of semiconductors are controlled by the nature and density of carriers,and the location of the Fermi energy.Developing strategies to tune these parameters holds the key to precise control over semiconductors properties.We propose that ligand exchange on superatoms can offer a systematic strategy to vary these properties.We demonstrate this by considering a WSe_(2) surface doped with ligated metal chalcogenide Co_(6)Se_(8)(PEt_(3))_(6) clusters.These superatoms are characterized by valence quantum states that can readily donate multiple electrons.We find that the WSe_(2) support binds more strongly to the Co_(6)Se_(8) cluster than the PEt3 ligand,so ligand exchange between the phosphine ligand and the WSe_(2) support is energetically favorable.The metal chalcogenide superatoms serves as a donor that may transform the WSe_(2) p-type film into an ntype semiconductor.The theoretical findings complement recent experiments where WSe_(2) films with supported Co_(6)Se_(8)(PEt_(3))_(6) are indeed found to undergo a change in behavior from p-to n-type.We further show that by replacing the PEt3 ligands by CO ligands,one can control the electronic character of the surface and deposited species.

Stacking monolayers at will:A scalable device optimization strategy for two-dimensional semiconductors

In comparison to monolayer(1L),multilayer(ML)two-dimensional(2D)semiconducting transition metal dichalcogenides(TMDs)exhibit more application potential for electronic and optoelectronic devices due to their improved current carrying capability,higher mobility,and broader spectral response.However,the investigation of devices based on wafer-scale ML-TMDs is still restricted by the synthesis of uniform and high-quality ML films.In this work,we propose a strategy of stacking MoS_(2) monolayers via a vacuum transfer method,by which one could obtain wafer-scale high-quality MoS_(2) films with the desired number of layers at will.The optical characteristics of these stacked ML-MoS_(2) films(>2L)indicate a weak interlayer coupling.The stacked MLMoS_(2) phototransistors show improved optoelectrical performances and a broader spectral response(approximately 300-1,000 nm)than that of 1L-MoS_(2).Additionally,the dual-gate ML-MoS_(2) transistors enable enhanced electrostatic control over the stacked ML-MoS_(2) channel,and the 3L and 4L thicknesses exhibit the optimal device performances according to the turning point of the current on/off ratio and the subthreshold swing.

Oxidations of two-dimensional semiconductors:Fundamentals and applications

Since the discovery of graphene,two-dimensional(2D)semiconductors have been attracted intensive interest due to their unique properties.They have exhibited potential applications in next generation electronic and optoelectronic devices.However,most of the 2D semiconductor are known to suffer from the ambient oxidation which degrade the materials and therefore hinder us from the intrinsic materials’properties and the optimized performance of devices.In this review,we summarize the recent progress on both fundamentals and applications of the oxidations of 2D semiconductors.We begin with the oxidation mechanisms in black phosphorus,transition metal dichalcogenides and transition metal monochalcogenides considering the factors such as oxygen,water,and light.Then we show the commonly employed passivation techniques.In the end,the emerging applications utilizing controlled oxidations will be introduced.

Bandgap engineering of high mobility two-dimensional semiconductors toward optoelectronic devices

Over the last few years,great advances have been achieved in exploration of high-mobility two-dimensional(2D)semiconductors such as metal chalcogenide InSe and noble-transition-metal dichal-cogenide PdSe_(2).These materials are competitive candidates for constructing next-generation optoelec-tronic devices owing to their unique crystalline and electronic structures.Moreover,the optical and electronic properties of 2D materials can be efficiently modified via precisely engineering their band structures,which is critical for widening specific applications ranging from high-performance opto-electronics to catalysis and energy harvesting.In this review,we focus on the progress in bandgaps engineering of newly emerging high-mobility 2D semiconductors and their applications in optoelec-tronic devices,incorporating our recent study in the InSe and PdSe_(2)systems.First of all,we discuss the structure-property relationship of typical high-mobility 2D semiconductors(InSe and PdSe 2).Next,we analyze several viable strategies for bandgap engineering,including thickness,strain or pressure,alloying,heterostructure,surface modification,intercalation,and so on.Furthermore,we summarize the optoelectronic devices fabricated with such high-mobility 2D semiconductors.The conclusion and outlook in this topic are finally presented.This review aims to provide valuable insights in bandgap engineering of newly emerging 2D semiconductors and explore their potential in future optoelectronic applications.

Two-dimensional transition metal halide PdX_(2)(X=F,Cl,Br,I):A promising candidate of bipolar magnetic semiconductors

Two-dimensional(2D)nanomaterials with bipolar magnetism show great promise in spintronic applications.Manipulating carriers'spin-polarized orientation in bipolar magnetic semiconductor(BMS)requires a gate voltage,but that is volatile.Recently,a new method has been proposed to solve the problem of volatility by introducing a ferroelectric gate with proper band alignment.In this paper,we predict that the PdX_(2)(X=F,Cl,Br,I)monolayers are 2D ferromagnetic BMS with dynamic stability,thermal stability,and mechanical stability by first-principles calculations.The critical temperatures are higher than the boiling point of liquid nitrogen and the BMS characteristics are robust against external strains and electric fields for PdCl_(2) and PdBr_(2).Then,we manipulate the spin-polarization of PdCl_(2) and PdBr_(2) by introducing a ferroelectric gate to enable magnetic half-metal/semiconductor switching and spin-up/down polarization switching control.Two kinds of spin devices(multiferroic memory and spin filter)have been proposed to realize the spin-polarized directions of electrons.These results demonstrate that PdCl_(2) and PdBr_(2) with BMS characters can be widely used as a general material structure for spintronic devices.

Boosting MA-based two-dimensional Ruddlesden-Popper perovskite solar cells by incorporating a binary spacer

Two-dimensional Ruddlesden-Popper(2DRP)perovskite exhibits excellent stability in perovskite solar cells(PSCs)due to introducing hydrophobic long-chain organic spacers.However,the poor charge transporting property of bulky organic cation spacers limits the performance of 2DRP PSCs.Inspired by the Asite cation alloying strategy in 3D perovskites,2DRP perovskites with a binary spacer can promote charge transporting compared to the unary spacer counterparts.Herein,the superior MA-based 2DRP perovskite films with a binary spacer,including 3-guanidinopropanoic acid(GPA)and 4-fluorophenethylamine(FPEA)are realized.These films(GPA_(0.85)FPEA_(0.15))_(2)MA_(4)Pb_5I_(16)show good morphology,large grain size,decreased trap state density,and preferential orientation of the as-prepared film.Accordingly,the present 2DRP-based PSC with the binary spacer achieves a remarkable efficiency of 18.37%with a V_(OC)of1.15 V,a J_(SC)of 20.13 mA cm^(-2),and an FF of 79.23%.To our knowledge,the PCE value should be the highest for binary spacer MA-based 2DRP(n≤5)PSCs to date.Importantly,owing to the hydrophobic fluorine group of FPEA and the enhanced interlayer interaction by FPEA,the unencapsulated 2DRP PSCs based on binary spacers exhibit much excellent humidity stability and thermal stability than the unary spacer counterparts.

Two-Dimensional Perovskite Single Crystals for High-Performance X-ray Imaging and Exploring MeV X-ray Detection

Scintillation semiconductors play increasingly important medical diagnosis and industrial inspection roles.Recently,two-dimensional(2D)perovskites have been shown to be promising materials for medical X-ray imaging,but they are mostly used in low-energy(≤130 keV)regions.Direct detection of MeV X-rays,which ensure thorough penetration of the thick shell walls of containers,trucks,and aircraft,is also highly desired in practical industrial applications.Unfortunately,scintillation semiconductors for high-energy X-ray detection are currently scarce.Here,This paper reports a 2D(C_(4)H_(9)NH_(3))_(2)PbBr_(4)single crystal with outstanding sensitivity and stability toward X-ray radiation that provides an ultra-wide detectable X-ray range of between 8.20 nGy_(air)s^(-1)(50 keV)and 15.24 mGy_(air)s^(-1)(9 MeV).The(C_(4)H_(9)NH_(3))_(2)PbBr_(4)single-crystal detector with a vertical structure is used for high-performance X-ray imaging,delivering a good spatial resolution of 4.3 Ip mm^(-1)in a plane-scan imaging system.Low ionic migration in the 2D perovskite enables the vertical device to be operated with hundreds of keV to MeV X-ray radiation at high bias voltages,leading to a sensitivity of 46.90μC Gy_(air)-1 cm^(-2)(-1.16 Vμm^(-1))with 9 MeV X-ray radiation,demonstrating that 2D perovskites have enormous potential for high-energy industrial applications.

High-Performance Organic Field-Effect Transistors Based on Two-Dimensional Vat Orange 3 Crystals

The exploration and research of low-cost,environmentally friendly,and sustainable organic semiconductor materials are of immense significance in various fields,including electronics,optoelectronics,and energy conversion.Unfortunately,these semiconductors have almost poor charge transport properties,which range from∼10^(−4) cm^(2)·V^(−1)·s^(−1) to∼10^(−2) cm^(2)·V^(−1)·s^(−1).Vat orange 3,as one of these organic semiconductors,has great potential due to its highly conjugated structure.We obtain high-quality multilayered Vat orange 3 crystals with two-dimensional(2D)growth on h-BN surfaces with thickness of 10–100 nm using physical vapor transport.Raman’s results confirm the stability of the chemical structure of Vat orange 3 during growth.Furthermore,by leveraging the structural advantages of 2D materials,an organic field-effect transistor with a 2D vdW vertical heterostructure is further realized with h-BN encapsulation and multilayered graphene contact electrodes,resulting in an excellent transistor performance with On/Off ratio of 104 and high field-effect mobility of 0.14 cm^(2)·V^(−1)·s^(−1).Our results show the great potential of Vat orange 3 with 2D structures in future nano-electronic applications.Furthermore,we showcase an approach that integrates organic semiconductors with 2D materials,aiming to offer new insights into the study of organic semiconductors.