Ultrathin Crystal Research Articles

Super High-k Unit-Cell-Thick α-CaCr2O4 Crystals.

High-dielectric-constant (high-k) insulators are indispensable components to integrate semiconductors into metal-oxide-semiconductor field-effect transistors with sub-10 nm channel length, where the equivalent oxide thickness (EOT) of high-k insulator needs to be decreased to subnanometer scale. The traditional insulators, including Al2O3, SiO2, and HfO2, fit well with the existing silicon industry but suffer from serious degeneration of insulating properties, such as large leakage currents caused by high-density borders and interface traps, when their thicknesses are reduced to a few nanometers. Here, we synthesize a high-quality nonlayered ultrathin α-CaCr2O4 crystal down to unit-cell thickness (∼1.2 nm) by an elements slow-supply chemical vapor deposition (CVD) method. The unit-cell-thick α-CaCr2O4 crystals show a super high dielectric constant of 87.34, which is over 20 times higher than that of well-known layered insulator h-BN and corresponds to an EOT below 1 nm. Furthermore, it has a high breaking strength (39 GPa) and excellent stability. This strategy can also be used to fabricate other ultrathin ternary oxides, such as high-k ultrathin FeNb2O6 crystals, demonstrating the universality of the CVD method.

Synthesis of Ultrathin FeS Nanosheets via Chemical Vapor Deposition.

Fe-based 2D materials exhibit rich chemical compositions and structures, which may imply many unique physical properties and promising applications. However, achieving controllable preparation of ultrathin non-layered FeS crystal on SiO2/Si substrate remains a challenge. Herein, the influence of temperature and molecular sieves is reported on the synthesis of ultrathin FeS nanosheets with a thickness as low as 2.3nm by molecular sieves-assisted chemical vapor deposition (CVD). The grown FeS nanosheets exhibit a non-layered hexagonal NiAs structure and belong to the P63/mmc space group. The inverted symmetry broken structure is confirmed by the angle-resolved second harmonic generation (SHG) test. In particular, the 2D FeS nanosheets exhibit exceptional metallic behavior, with conductivity up to 1.63 × 106S m-1 at 300K for an 8nm thick sample, which is higher than that of reported 2D metallic materials. This work provides a significant contribution to the synthesis and characterization of 2D non-layered Fe-based materials.

An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation.

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.

Ultrathin photonic crystal based on photo-crosslinked polymer and metal–organic framework for highly sensitive detection and discrimination of benzene series vapors

Volatile organic compounds (VOCs) have always been a major concern as a global environmental problem. As a low-cost, high-efficiency and visual sensor, photonic crystals (PCs) have been actively studied in VOCs detection. Herein, a one-dimensional PC sensor for visual sensing of highly toxic benzene series VOC vapors is prepared for the first time by integrating a new photo-crosslinked polymer-poly(styrene-benzoylphenyl acrylate) P(St-BPA) and a high specific surface area metal–organic framework (MOF) MIL-101(Cr). The PC can detect VOCs quantitatively and visually, and clearly distinguish 7 benzene series vapors. The detection limit of the benzene series VOCs is as low as 0.06–3.45 g/m3. Meanwhile, owing to the ultra-thin layer and porous structure, the PC can reach a response equilibrium to the VOCs within 1–2.6 s. Moreover, the PC has a good organic vapor tolerance and can maintain stable optical performance after 1000 times of reuse in VOCs. Besides, 4 other PCs assembled with different aryl polymers and MOFs are first fabricated and their sensing performance to benzene series VOCs are studied and compared, which provides a valuable reference for the selection of materials for the preparation of such PC sensors.

Wide-angle and Fano-shape contrast between emission and absorption in hybrid polariton-involved planar structure

Nonreciprocal light propagation in planar structure has become a hot topic because of the pattern-free fabrication process and the potentials for exploring heat transfer and energy management. An effective strategy incorporated with natural van der Waals material (α-phase molybdenum trioxide, α-MoO3), ultra-thin polar crystal (silicon carbide, SiC) thin film and a Weyl semimetal interlayer sitting on a metal (silver, Ag) substrate is proposed. The emission and absorption of the suggested device is explored with electromagnetic simulations that utilize the 4×4 transfer matrix method. Angular-resolved emission shows that the Fano-shape of nonreciprocity can be maintained within wide angle range. Tunable Fano resonances with different line shapes are demonstrated by varying the structural parameters. Moreover, the enhanced nonreciprocal radiation effect can be observed for both s- and p-polarized incidence waves. The proposal allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared emitters.

An Ultrathin Inorganic Molecular Crystal Interfacial Layer for Stable Zn Anode.

Zn metal anode suffers from dendrite growth and side reactions during cycling, significantly deteriorating the lifespan of aqueous Zn metal batteries. Herein, we introduced an ultrathin and ultra-flat Sb2O3 molecular crystal layer to stabilize Zn anode. The in-situ optical and atomic force microscopes observations show that such a 10nm Sb2O3 thin layer could ensure uniform under-layer Zn deposition with suppressed tip growth effect, while the traditional WO3 layer undergoes an uncontrolled up-layer Zn deposition. The superior regulation capability is attributed to the good electronic-blocking ability and low Zn affinity of the molecular crystal layer, free of dangling bonds. Electrochemical tests exhibit Sb2O3 layer can significantly improve the cycle life of Zn anode from 72 h to 2800 h, in contrast to the 900 h of much thicker WO3 even in 100 nm. This research opens up the application of inorganic molecular crystals as the interfacial layer of Zn anode.

Supersymmetry Laser Arrays with High‐Order Exceptional Point

Supersymmetry (SUSY) laser array with superpartner structure can suppress excess modes to achieve high‐intensity and high‐coherent radiation. Compared with complex superpartner, using parity time (PT) symmetry broken to manipulate SUSY laser arrays is a more flexible approach. Herein, based on ultrathin perovskite single crystal, a SUSY laser array is constructed without superpartner, but with auxiliary gain–loss structure. PT symmetry broken with high‐order exceptional point (EP) is realized in the visible spectral range. It intrinsically avoids superpartner field mismatch and improves side mode suppression ratio 8.8–12 dB for single‐mode lasing. Furthermore, variations in EP‐order enable the transition from single‐mode to dual‐mode or tri‐mode radiation and retain amplified radiation characteristics. In addition, these SUSY laser arrays with the high‐order EP have the potential to be small‐volume optical sensor devices. The new design combining SUSY and PT symmetry broken presents its potential in micro‐nanophotonic devices with the benefit of small size, flexibility in spectral control, expandability, simplicity, and multifunctionality feature.

Ultrathin GeTe Crystal in a Strong Femtosecond Laser Field: Manifestation of a Quantum Size Effect

The behavior of a thin-film GeTe crystal induced by intense femtosecond laser pulses (μm) has been studied using a pulsed electron diffractometer. The sample is an annealed 20-nm GeTe film on a copper grid with a carbon coating. It has been found that laser ablation results in the formation of an ultrathin GeTe crystal (assumingly, GeTe monolayer) with a high radiation resistance. Possible reasons for the detected nanosize effect are discussed.

Ultrathin GeTe Crystal in a Strong Femtosecond Laser Field: Manifestation of a Quantum Size Effect

The behavior of a thin-film GeTe crystal induced by intense femtosecond laser pulses (lambda = 0.8{kern 1pt} μm) has been studied using a pulsed electron diffractometer. The sample is an annealed 20-nm GeTe film on a copper grid with a carbon coating. It has been found that laser ablation results in the formation of an ultrathin GeTe crystal (assumingly, GeTe monolayer) with a high radiation resistance. Possible reasons for the detected nanosize effect are discussed.

Single‐Orientation Epitaxy of Quasi‐1D Tellurium Nanowires on M‐Plane Sapphire for Highly Uniform Polarization Sensitive Short‐Wave Infrared Photodetection

AbstractTellurium (Te), an elemental van der Waals semiconductor, has intriguing anisotropic physical properties owing to its inherent quais‐1D crystal structure. Synthesizing ultrathin Te crystal with uniform orientation is important to its large‐scale device applications, but that remains a great challenge. Herein, the nanoscale grooves‐induced unidirectional epitaxy growth of 1D Te nanowires via physical vapor deposition on the annealed m‐plane sapphire is demonstrated. By enhancing the annealing temperature from 1000 to 1300 °C, nanoscale grooves on m‐plane sapphire arising along the [100] direction and gradually distinct, and the corresponding Te nanowires grown on them turns from random to uniform, finally achieving nearly 95% unidirectional Te nanowires. The as‐grown Te nanowires possess high crystallinity with clearly chiral helical chains along the c‐axis direction and reveal thickness‐tunable bandgap with prominent linear‐dichroic. As results, the Te nanowire‐based photodetectors demonstrate a broadband photoresponse from visible (532 nm) to short‐wave infrared (2530 nm), with high responsivity of 327 A W−1 as well as strong and uniform polarization sensitivity (anisotropic ratio = 2.05) to 1550 nm light. The high crystallinity and superior anisotropy of Te nanowires, combined with the orientation‐controlled preparation endows it with great feasibility for constructing chip‐scale multifunctional optoelectronic devices.

Controllable Oxidation of ZrS2 to Prepare High-κ, Single-crystal m-ZrO2 for 2D Electronics.

High-κ materials that exhibit large permittivity and band gaps are needed as gate dielectrics to enhance capacitance and prevent leakage current in downsized technology nodes. Among these, monoclinic ZrO2 (m-ZrO2 ) shows good potential because of its inertness and high-κ with respect to SiO2 , but a method to produce ultrathin single crystal is lacking. Here, the controllable preparation of ultrathin m-ZrO2 single crystals via the in situ thermal oxidation of ZrS2 is achieved. As-grown m-ZrO2 presents an equivalent oxide thickness of ≈0.29nm, a high dielectric constant of ≈19, and a breakdown voltage (EBD ) of ≈7.22 MV cm-1 . MoS2 field effect transistor (FET) by using m-ZrO2 as a dielectric layer shows comparable mobility to that using SiO2 dielectric. The ultraclean interface of m-ZrO2 /MoS2 and high crystalline quality of m-ZrO2 lead to negligible hysteresis in transfer curves. Single crystal m-ZrO2 dielectric shows potential application in digital complementary metal oxidesemiconductor (CMOS) logic FET.

Liquid Crystal Elastomer Based Dexterous Artificial Motor Unit.

Despite the great advancement in designing diverse soft robots, they are not yet as dexterous as animals in many aspects. One challenge is that they still lack the compact design of an artificial motor unit with a great comprehensive performance that can be conveniently fabricated, although many recently developed artificial muscles have shown excellent properties in one or two aspects. Herein, an artificial motor unit is developed based on gold-coated ultrathin liquid crystal elastomer (LCE) film. Subject to a voltage, Joule heating generated by the gold film increases the temperature of the LCE film underneath and causes it to contract. Due to the small thermal inertial and electrically controlling method of the ultrathin LCE structure, its cyclic actuation speed is fast and controllable. It is shown that under electrical stimulation, the actuation strain of the LCE-based motor unit reaches 45%, the strain rate reaches 750%/s, and the output power density is as high as 1360Wkg-1 . It is further demonstrated that the LCE-based motor unit behaves like an actuator, a brake, or a nonlinear spring on demand, analogous to most animal muscles. Finally, as a proof-of-concept, multiple highly dexterous artificial neuromuscular systems are demonstrated using the LCE-based motor unit.

Layer transfer of ultrathin Ge crystal segregated on Al/Ge(111) structure

Effects of the surface modification by O2 plasma exposure on the Al/Ge(111) structure have been investigated in order to get an insight into a layer transfer technique of the ultrathin Ge layer segregated on the Al/Ge(111) structure toward the device fabrication, and then the wafer bonding of the Al/Ge(111) structure to the thermally-grown SiO2/Si structure has been demonstrated. The O2 plasma treatment and the subsequent pure water rinse were found to be effective to form the hydrophilic surface of the Al/Ge(111) structure with a suppression of the segregated Ge layer oxidation. The Al/Ge(111) structure with the hydrophilic surface was then bonded to the SiO2/Si substrate, and its bonding strength was enough to perform Ge thinning by the chemical mechanical polishing and the wet-chemical etching using H2O2-based solutions. Ohmic contact of the ring-type device pattern with the segregated Ge/Al stack was achieved by using the remaining p-type Ge substrate as the contact pads.

Individual and synergetic charge transport properties at the solid and electrolyte interfaces of a single ultrathin single crystal of organic semiconductors.

The chemical structures and morphologies of organic semiconductors (OSCs) and gate dielectrics have been widely investigated to improve the electrical performances of organic thin-film transistors (OTFTs) because the charge transport therein is a phenomenon at the semiconductor-dielectric interfaces. Here, solid and ionic gel gate dielectrics were adopted on the lower and upper surfaces, respectively, of a single, two molecule-thick single crystals of p-type OSCs to study the charge transport properties at individual interfaces between the morphologically compatible OSC surface and different gate dielectrics. Using the four-probe method, the solid and ionic gel interfaces were found to exhibit hole mobilities of 9.3 and 2.2 cm2 V-1 s-1, respectively, which revealed the crucial impact of the gate dielectric materials on the interfacial charge transport. Interestingly, when gate biases are applied through both dielectrics, i.e., under the solid/ionic gel dual-gate transistor operation, the hole mobility at the solid gate interface is improved up to 14.7 cm2 V-1 s-1, which is 1.5 times greater than that assessed without the ionic gel gate. This improvement can be attributed to the electric double layer formed at the ionic gel/uniform crystal surface, which provides a close-to-ideal charge transport interface through dramatic trap-filling. Therefore, the present dual-gate transistor technique will be promising for investigating the intrinsic charge-transport capabilities of OSCs.

A New Class of Organic Crystals with Extremely Large Hyperpolarizability: Efficient THz Wave Generation with Wide Flat‐Spectral‐Band

AbstractIn organic π‐conjugated crystals, enhancing molecular optical nonlinearity of chromophores (e.g., first hyperpolarizability β ≥ 300 × 10−30 esu) in most cases unfortunately results in zero macroscopic optical nonlinearity, which is a bottleneck in organic nonlinear optics. In this study, a new class of nonlinear optical organic crystals introducing a chromophore possessing an extremely large first hyperpolarizability is reported. With newly designed 4‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐1‐(pyrimidin‐2‐yl)pyridin‐1‐ium (PMPR) chromophore, incorporating a head‐to‐tail cation‐anion OH⋯O hydrogen‐bonding synthon and an optimal selection of molecular anion into crystals results in extremely large macroscopic optical nonlinearity with effective first hyperpolarizability of 335 × 10−30 esu. This is in sharp contrast to zero value for previously reported analogous crystals. An ultrathin PMPR crystal with a thickness of ≈10 µm exhibits excellent terahertz (THz) wave generation performance. Both i) broadband THz wave generation with a wide flat‐spectral‐band in the range of 0.7–3.4 THz defined at −3 dB and high upper cut‐off generation frequency of > 7 THz as well as ii) high‐generation efficiency (5 times higher THz amplitude than ZnTe crystal with a mm‐scale thickness) are simultaneously achieved. Therefore, new PMPR crystals are highly promising materials for diverse applications in nonlinear optics and THz photonics.

High-dimensional orbital angular momentum entanglement from an ultrathin nonlinear film

Entanglement, as a crucial feature of quantum systems, is essential for various applications of quantum technologies. High-dimensional entanglement has the potential to encode arbitrary large amount of information and enhance robustness against eavesdropping and quantum cloning. The orbital angular momentum (OAM) entanglement can achieve the high-dimensional entanglement nearly for free stems due to its discrete and theoretically infinite-dimensional Hilbert space. A stringent limitation, however, is that the phase-matching condition limits the entanglement dimension because the coincidence rate decreases significantly for high-order modes. Here we demonstrate relatively flat high-dimensional OAM entanglement based on a spontaneous parametric down conversion (SPDC) from an ultrathin nonlinear lithium niobite crystal. The difference of coincidences between the different-order OAM modes significantly decreases. To further enhance the nonlinear process, this microscale SPDC source will provide a promising and integrated method to generate optimal high-dimensional OAM entanglement.

Nonlinear Optical Imaging of In‐Plane Anisotropy in Two‐Dimensional SnS (Advanced Optical Materials 10/2022)

This cover illustration represents a laser field interacting with an ultrathin SnS crystal and the production of a second harmonic generation (SHG) field. Two representative polar diagrams of the polarization-resolved SHG intensity from different SnS crystals, and the fitting with the nonlinear optics model, are also shown, enabling the extraction of quantitative information on the in-plane anisotropy in SnS. For further details see article number 2102776 by Emmanuel Stratakis and co-workers.

Fabrication and transport properties of two dimensional Bi2Sr2Ca2Cu3O10+δ micro-bridge

Ultra-thin high-temperature superconducting films have attracted continuous interest due to their potential electronic applications, which also provide a unique platform of novel physics and properties in the two-dimensional limit. We, here, realized fabrication of two-unit-cell-thick micro-bridges from mechanically exfoliated ultra-thin Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals and systematically investigated their transport properties. The two-dimensional superconducting nature is verified by the existence of the Berezinskii–Kosterlitz–Thouless transition, which is simultaneously revealed by current-voltage properties and the zero-field temperature dependence of resistance. Comparing with Bi2223 bulk crystal, a Bi2223 micro-bridge shows a slight lower upper critical field but pronounced improvement in the critical current density. Our findings indicate that the ultra-thin Bi2223 single crystal is highly prospective for both scientific investigations of unconventional superconductivity and applications of high Tc superconducting devices.

Emittance preserving thin film plasma mirrors for GeV scale laser plasma accelerators

Laser-plasma accelerators (LPAs) now routinely produce electron beams with GeV energies over acceleration lengths on the order of a few centimeters. This capability and the demonstration of multistage coupling make LPAs promising for numerous applications. However, beam quality preservation in multistage accelerators remains an obstacle because of the need to separate the laser pulse from the electron beam. Plasma mirrors can be used to redirect the laser pulse, but their substrate thickness threatens to substantially degrade the electron beam emittance. Ultrathin ($\ensuremath{\sim}20\text{ }\text{ }\mathrm{nm}$) liquid crystal (LC) plasma mirrors are an excellent candidate to address this challenge. This work investigates the robustness of thin LC plasma mirrors in the presence of capillary discharge plasma and an auxiliary heater laser. We find they can be operated $\ensuremath{\sim}10\text{ }\text{ }\mathrm{cm}$ from the capillary exit when a heater laser is used. We then performed a normalized emittance measurement enabled using a 20 nm LC plasma mirror to protect electron beam optics after the LPA. The emittance contribution from scattering through the plasma mirror is calculated to be of order 100 nm, much less than the measured emittance of $4.0\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Finally, we develop a model to calculate plasma mirror performance based on the laser polarization and intensity, and plasma mirror thickness.

Damage Control of Ultra-Thin YAG Crystal in Nano-Machining Process

Yttrium aluminum garnet (YAG) crystal is one of the most widely used laser crystals because of its excellent properties. Thin disk laser with ultra-thin YAG crystal as gain medium is widely used in micromachining, imaging, military, medical, scientific research, and so on. Ultra-thin crystals with nanometer precision surface quality are easily damaged in nano-machining process. From the fracture mechanics, the critical fracture stress of ultra-thin YAG crystal was analyzed. The model of ultra-thin YAG crystal in chemical mechanical polishing (CMP) process, one of important nano-machining technologies, was established, and the effect of sticking mode and pressure on the stress magnitude and distribution of crystal were investigated. It was confirmed that the damage phenomenon in CMP process can be effectively controlled by using the edge padding sticking method. The optimized process parameters are as follows: the pressure should be less than 65 kPa, the optimized size of the edge pad crystal for YAG crystal with 10 mm × 10 mm is 10 mm × 20 mm, and the right-angle damage limit of the edge padding crystal is 3 mm. Finally, the optimization results were experimentally verified and the high-efficiency and high-quality nano-machining of ultra-thin YAG crystal with surface roughness S a 2.85 nm and the thickness 0.175 mm was achieved.