Two-dimensional Single Crystals Research Articles

Superconducting Field-Effect Transistors with PdxTe-Te Intimate Contacts.

Superconducting-based electronic devices have shown great potential for future quantum computing applications. One key building block device is a superconducting field-effect transistor based on a superconductor-semiconductor-superconductor Josephson-junction (JJ) with a gate-tunable semiconducting channel. However, the performance of such devices is highly dependent on the quality of the superconductor to semiconductor interface. In this study, we present an alternative method to obtain a high-quality interface by using intimate contact. We investigate the proximity-induced superconductivity in chiral crystal tellurium (Te) and fabricate a PdxTe-Te-PdxTe JJ with an ambipolar supercurrent that is gate-tunable and exhibits multiple Andreev reflections. The semiconducting two-dimensional Te single crystal is grown hydrothermally and partially converted to superconducting PdxTe by controlled annealing. Our work demonstrates a promising path for realizing controllable superconducting electronic devices with high-quality superconducting interfaces; thus, we can continue to advance the field of quantum computing and other interface-based technologies.

Controlled growth of 3R phase niobium diselenide and its properties

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6–7 μs, 0.5 ms − 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

Shape-Dependent Optical Waveguides and Low-Threshold Lasers from Polymorphic Two-Dimensional Organic Single Crystals.

Organic single crystals (OSCs) with uniform morphologies and highly ordered molecular aggregations are promising for high-performance optoelectronic devices, such as organic solid-state lasers (OSSLs), organic light-emitting transistors (OLETs), and organic light-emitting diodes (OLEDs). However, manipulating OSC morphologies and aggregation is challenging. In this study, we synthesized two-dimensional (2D) OSCs of 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) in hexagonal and parallelogram microplate (H-MP and P-MP) forms. Both types exhibit H-aggregation in the 2D plate plane but with different molecular transition dipole moment (TDM) orientations. This leads to different photon coupling modes with H-MP and P-MP microcavities. H-MPs enable isotropic 2D-waveguiding, forming whispering gallery mode (WGM) resonators, while P-MPs create unidirectional waveguiding, forming Fabry-Pérot mode (FP) resonators. These resonators can generate low-threshold laser emissions at 467 and 473 nm, respectively, and exhibit superior lasing stability with a half-life exceeding 2 h. Our BSBCz microplate OSCs are attractive candidates to combine controlled organic microcavities with photon transporting for realizing future integrated optoelectronic devices.

Surface doping effect on the optoelectronic performance of 2D organic crystals based on cyano-substituted perylene diimides

Compared to organic thin films, organic single crystals offer significant potential in organic phototransistors (OPTs) due to their enhanced charge transport, large surface area, and defect-free nature. However, the development of n-type semiconductors has lagged behind p-type semiconductors. To enhance semiconductor device performance, a doping process can be employed, which typically involves the introduction of charged impurities into the crystalline semiconducting material. Its aim is to reduce the Ohmic losses, increase carrier density, improve transport capabilities, and facilitate effective carrier injection, ultimately enhancing the electrical properties of the material. Traditional doping processes, however, often pose a risk of damaging the structure of single crystals. In this study, we have synthesized novel cyano-substituted chiral perylene diimides, which self-assemble into two-dimensional single crystals that can be used for n-type semiconductor devices. We have employed a surface doping strategy using diethylamine vapor without disrupting the crystal structure. The fabricated devices exhibit significantly higher charge transport properties after doping, achieving a maximum electron mobility of 0.14 cm² V−¹ s−¹, representing an improvement of over threefold. Furthermore, the optoelectronic performance of the doped devices has significantly improved, with the external quantum efficiency increased by over 9 times and the significantly improved response time. These results suggest that our surface doping technology is a promising way for enhancing the performance of 2D organic single-crystal OPTs.

Guanidinium lead iodide two-dimensional perovskite single crystal for efficient x-ray detection

The recently reported two-dimensional lead halide perovskites have attracted much attention in high-energy radiation detector applications due to its high structural stability and low ion migration. However, the increased exciton binding energy and the charge transport barrier resulting from the A-site cation can easily limit the charge extraction and device performance. As an emerging material in high-energy detection field, guanidinium {GA+ = [C(NH2)3]+} is a relatively large organic cation composed of three amino groups, which can enhance the hydrogen bond formation and the van der Waals interactions with the metal halide framework. Meanwhile, the polymer symmetry of GA+ cation leads to almost zero dipole moments, excellent electronic properties, and high thermodynamic stability. However, there is a lack of report on the preparation, structure, and properties of GA-based 2D perovskite single crystals as well as their applications in x-ray detection. Herein, we have synthesized large-size two-dimensional GA2PbI4 single crystals with a bandgap of 2.28 eV by cooling crystallization and applied them to x-ray detectors. GA2PbI4 single crystals exhibit a high μτ product of 7.5 × 10−4 cm2 V−1 and a low trap density of 6.44 × 108 cm−3. An effective x-ray photoresponse was realized with a sensitivity as high as 392.4 μC Gyair−1 cm−2. The unique structure and excellent charge transport properties of GA+ organic cations exhibit promising prospects in high-performance x-ray detection.

Dual-band photoluminescence mechanism of magnetic doped two-dimensional (PEA)2PbBr4 single crystals

Mn2+ doping effectively realizes white light emission in three-dimensional lead halide perovskite nanocrystals. Meanwhile, research on Mn-doped two-dimensional layered perovskite single crystal is limited. We report centimeter-scale Mn-doped PEA2PbBr4 (C6H5CH2CH2NH3+ and PEA+) single crystals prepared by a slow evaporation method. Mn2+ dopants mainly act as substitutional doping and exhibit paramagnetic properties in the crystal at low doping density, while interstitial doping of Mn2+ prevails and induces antiferromagnetic characteristics at high doping density. Mn:PEA2PbBr4 single crystals exhibit dual-band chromacity-tunable blue-orange photoluminescence originating from excitons and Mn2+ emission. The negative temperature quenching effect is achieved by Mn-doping defects for the temperature-dependent exciton photoluminescence. Upon testing in the low-pressure vacuum chamber, the Mn2+ peak of the single crystal shows a dramatic shift from 610 to 690 nm. These results indicate that Mn:PEA2PbBr4 single crystal can serve as a potential and promising luminescent device material that achieves color tunable properties by regulating the systematic changes in the intensity ratio of exciton emission and Mn2+ emission, which will be very helpful for exploring the application of perovskite in magneto-optical devices in the future.

Emergent Intrinsic Ferromagnetism in Two-Dimensional Trigonal Rhodium Oxide.

Two-dimensional (2D) van der Waals single crystals with long-range magnetic order are the precondition and urgent task for developing a 2D spintronics device. In contrast to graphene and transition metal dichalcogenides, the study of 2D single-crystal metal oxides with intrinsic ferromagnetic properties remains a huge challenge. Here, we report a large-size trigonal single-crystal rhodium oxide (SC-Tri-RhO2), with crystal parameters of a = b = 3.074 Å, c = 6.116 Å, and a space group of P3̅m1 (164), exhibiting strong ferromagnetism (FM) at a rather high temperature. Furthermore, theoretical calculations suggest that the ferromagnetism in SC-Tri-RhO2 originates from spin splitting near the Fermi level, and the total magnetic moment is contributed mainly by the Rh atom.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Reducing Contact Resistance in Organic Field-Effect Transistors: A Comprehensive Comparison between 2D and Microrod Single Crystals.

A comprehensive comparison of organic single crystals based on a single material but with different dimensions provides a unique approach to probe their carrier injection mechanism. In this report, both two-dimensional (2D) and microrod single crystals with the same crystalline structure of an identical thiopyran derivative, 7,14-dioctylnaphtho[2,1-f:6,5-f']bis(cyclopentane[b]thiopyran) (C8-SS), are grown on a glycerol surface with the space-confined method. Organic field-effect transistors (OFETs) based on the 2D C8-SS single crystal exhibit superior performance compared with those based on the microrod single crystal, particularly in their contact resistance (RC). It is demonstrated that the resistance of the crystal bulk in the contact region plays a key role in RC of the OFETs. Thus, among the 30 devices tested, the microrod OFETs typically appear contact-limited, whereas the 2D OFETs possess significantly reduced RC arising from the tiny thickness of the 2D single crystal. The 2D OFETs show high operational stability and high channel mobility up to 5.7 cm2/V·s. The elucidation of the contact behavior highlights the merits and great potential of 2D molecular single crystals in organic electronics.

Author Correction: On-water surface synthesis of charged two-dimensional polymer single crystals via the irreversible Katritzky reaction

Author Correction: On-water surface synthesis of charged two-dimensional polymer single crystals via the irreversible Katritzky reaction

Innentitelbild: Circularly Polarized Lasing from a Microcavity Filled with Achiral Single‐Crystalline Microribbons (Angew. Chem. 9/2023)

Circularly polarized lasing with high dissymmetry factors has been realized in an organic crystal-filled cavity without any chiral materials or structures, as reported by Qing Liao, Hongbing Fu, and co-workers in their Research Article (e202213229). The anisotropic refractive properties of two-dimensional organic single crystals lead to the Rashba–Dresselhaus effect inducing spin-splitting of cavity modes. The results provide a new idea for the design of CP lasers towards future 3D laser displays, information storage, and other fields.

Circularly polarized electroluminescence from a single-crystal organic microcavity light-emitting diode based on photonic spin-orbit interactions

Circularly polarized (CP) electroluminescence from organic light-emitting diodes (OLEDs) has aroused considerable attention for their potential in future display and photonic technologies. The development of CP-OLEDs relies largely on chiral-emitters, which not only remain rare owing to difficulties in design and synthesis but also limit the performance of electroluminescence. When the polarization (pseudospin) degrees of freedom of a photon interact with its orbital angular momentum, photonic spin-orbit interaction (SOI) emerges such as Rashba-Dresselhaus (RD) effect. Here, we demonstrate a chiral-emitter-free microcavity CP-OLED with a high dissymmetry factor (gEL) and high luminance by embedding a thin two-dimensional organic single crystal (2D-OSC) between two silver layers which serve as two metallic mirrors forming a microcavity and meanwhile also as two electrodes in an OLED architecture. In the presence of the RD effect, the SOIs in the birefringent 2D-OSC microcavity result in a controllable spin-splitting with CP dispersions. Thanks to the high emission efficiency and high carrier mobility of the OSC, chiral-emitter-free CP-OLEDs have been demonstrated exhibiting a high gEL of 1.1 and a maximum luminance of about 60000 cd/m2, which places our device among the best performing CP-OLEDs. This strategy opens an avenue for practical applications towards on-chip microcavity CP-OLEDs.

Epitaxial growth of wafer-scale single-crystal transition metal dichalcogenide monolayers for future electronics

The growth of wafer-scale two-dimensional (2D) single crystals, especially transition metal dichalcogenides (TMDs), is significantly essential for various electronic applications. However, the growth of wafer-scale single-crystal TMD (sc-TMD) monolayers on commercial substrates still remains challenging. Recently, two works published in <i>Nature Nanotechnology</i> have independently reported a substrate-cutting strategy for epitaxial growth of wafer-scale sc-TMD monolayers, which might open up a new route for preparing wafer-scale sc-TMD monolayers on commercially scalable insulating substrates for future electronics.

Ultrahigh yield and large-scale fast growth of large-size high-quality van der Waals transition-metal telluride single crystals

Stable, rapid, and large-scale production of high-quality, large-size two-dimensional (2D) van der Waals single crystals is an important prerequisite for realizing the potential of highly integrated 2D devices. Here, we report a chloride-mediated chemical vapor transport (CVT) approach to achieve large-scale production of MoTe 2 and WTe 2 single crystals with lateral sizes up to 2 cm and extremely high quality. The largest magnetoresistance and carrier mobility can reach 170% and 1,390.09 cm 2 V -1 s -1 for 2H-MoTe 2 crystals and 506,017% and 11,952.10 cm 2 V -1 s -1 for Td-WTe 2 crystals, respectively, and all are among the best reported ones. Compared with the conventional CVT method, which has a long growth cycle and much reactive reagent surplus, the growth speed of the current method is about 73 to 9,144 times faster, with ultrahigh yields of nearly 100%. The high crystallinity guarantees the crystals can be easily exfoliated to large-scale 2D crystals down to monolayers for assembling 2D high-performance devices. • High-yield and large-scale fast growth of van der Waals TMT single crystals • TMTs exhibit high crystalline quality and good electrical transport properties • TMTs monolayers with millimeter-scale lateral dimensions • TMT-based field-effect transistor and Hall device exhibit stable performance Obtaining high-quality, large-size 2D vdW single crystals is an important prerequisite for realizing the potential of highly integrated 2D devices. Here, Yang et al. report a chloride-mediated CVT approach to achieve ultrahigh-yield and large-scale fast growth of centimeter-sized transition-metal telluride single crystals with extremely high quality.

Progress and perspective on the growth of two-dimensional single crystals

Progress and perspective on the growth of two-dimensional single crystals

Controlled growth of two-dimensional InAs single crystals via van der Waals epitaxy

Controlled growth of two-dimensional InAs single crystals via van der Waals epitaxy

In-plane optical anisotropy of two-dimensional VOCl single crystal with weak interlayer interaction

Transition-metal oxyhalides MOX (M = Fe, Cr, V; O = oxygen, X = F, Cl, Br, I), an emerging type of two-dimensional (2D) van der Waals materials, have been both theoretically and experimentally demonstrated to possess unique electronic and magnetic properties. However, the intrinsic in-plane anisotropic properties of 2D VOCl still lacks in-depth research, especially optical anisotropy. Herein, a systematic Raman spectroscopic study is performed on VOCl single-crystal with different incident laser polarization at various temperatures. The polarized-dependent Raman scattering spectra reveal that the A g mode of VOCl show a 2-lobed shape in parallel polarization configuration while a 4-lobed shape in vertical configuration. In addition, the temperature-dependent and thickness-dependent Raman scattering spectra confirm a relatively weak van der Waals interaction between each layers among VOCl single crystal. These findings might provide better understanding on the in-plane anisotropic phenomenon in VOCl layers, thus will accelate further application of 2D single crystals for nanoscale angle-dependent optoelectronics.

Long carrier diffusion length in two-dimensional lead halide perovskite single crystals

<h2>Summary</h2> Ruddlesden-Popper (RP) perovskites are two-dimensional semiconductors for high-performance optoelectronic devices. In this work, we report a long in-plane carrier diffusion length in 2D RP perovskite single crystals probed by scanning photocurrent microscopy. Carrier diffusion lengths of 7–14 μm are observed when the number of PbI₆⁻² octahedra between organic spacers increases from 1 to 3. Using detailed light intensity and electric-field-dependent photocurrent measurements, we attribute the observed long diffusion length to the dominating dissociated free carrier transport. This is further validated by time-resolved photoluminescence measurements, where the decay lifetime increases in the presence of an electric field. From our experiments, we conclude that the in-plane transport in RP perovskites is efficient because of the partial free carrier generation, which overcomes strong excitonic effects. Our results suggest that semiconducting devices fabricated from RP perovskite single crystals can be as efficient as their 3D counterparts.

Atomic-scale manufacture of metre-sized two-dimensional single crystals by interfacial modulation

With the shrinkage of the chip feature size, the short-channel effect becomes more and more predominate. The development of new quantum materials for high-performance devices has become imperative for the current technological development. Two-dimensional (2D) materials, due to their excellent physical and chemical properties, are thought to be the promising candidate of quantum materials for achieving the high-end electronic and optoelectronic devices. Like the development of silicon-based chips, the wafer-scale device applications of 2D materials must be based on the fabrication of high-quality, large-size 2D single crystals. However, the existing manufacturing techniques of the well-studied bulk single crystals cannot be fully applied to the fabrication of 2D single crystals due to the interfacial characteristics of 2D materials. So far, single crystals of metre-sized graphene, decimetre-sized hBN and wafer-sized TMDCs have been successfully prepared by chemical vapor deposition, but the sizes of other 2D single crystals are still very limited and not in the same league as conventional semiconductor materials. Therefore, it is urgent to develop an effective preparation strategy for the manufacture of various 2D single crystals. In this review, we mainly overview the fabrication techniques for the meter-scale growth of 2D single crystals, and propose three key modulation aspects in the atomic-scale manufacture, i.e. the growth modulation of 2D single nucleus, the preparation of single-crystal substrates, and the alignment control of 2D single-crystal domains, in order to provide a universal method of fabricating the large-size 2D single crystals. Finally, the prospect of chip devices based on these high-quality large-size novel 2D single crystals is discussed, thereby paving the way for the future industrial applications of electronics and optoelectronics.

On-water surface synthesis of charged two-dimensional polymer single crystals via the irreversible Katritzky reaction

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) are classes of structurally defined crystalline polymeric materials with exotic physical and chemical properties. Yet, synthesizing 2DP and 2D COF single crystals via irreversible reactions remains challenging. Here we report the synthesis of charged 2DP (C2DP) single crystals through an irreversible Katritzky reaction, under pH control, on a water surface. The periodically ordered 2DPs comprise aromatic pyridinium cations and counter BF4− anions. The C2DP crystals, which are composed of linked porphyrin and pyrylium monomers (C2DP-Por), have a tunable thickness of 2–30 nm and a lateral domain size up to 120 μm2. Single crystals with a square lattice (a = b = 30.5 Å) are resolved by imaging and diffraction methods with near-atomic precision. Furthermore, the integration of C2DP-Por crystals in an osmotic power generator device shows an excellent chloride ion selectivity with a coefficient value reaching ~0.9 and an output power density of 4 W m−2, superior to those of graphene and boron nitride. Large single crystals of charged 2D polymers are synthesized on a water surface, under kinetic control, by the irreversible Katritzky reaction. The crystals can act as an anion-selective membrane for osmotic energy generation.