Controlled growth of MoS2 via surface-energy alterations.

Monolayer MoS2 in triangular configurations with rich edges or high-quality uniform films are either catalytically active for the hydrogen evolution reaction or flexible for functional electronic and optoelectronic devices. Here, we have experimentally discovered that these two types of MoS2 products can be selectively synthesized on graphene or sapphire substrates, which are associated with both different adsorption energy and diffusion-energy barrier for vapor precursors during growth. Our study not only provides insights into the on-surface synthesis of high-quality MoS2 monolayers, but also can be applied to the growth of vertically-stacked and large-scale in-plane lateral MoS2-graphene heterostructures.

Bolometric Effect in Bi2 O2 Se Photodetectors.

Bi2 O2 Se is emerging as a photosensitive functional material for optoelectronics, and its photodetection mechanism is mostly considered to be a photoconductive regime in previous reports. Here, the bolometric effect is discovered in Bi2 O2 Se photodetectors. The coexistence of photoconductive effect and bolometric effect is generally observed in multiwavelength photoresponse measurements and then confirmed with microscale local heating experiments. The unique photoresponse of Bi2 O2 Se photodetectors may arise from a change of hot electrons during temperature rises instead of photoexcited holes and electrons. Direct proof of the bolometric effect is achieved by real-time temperature tracking of Bi2 O2 Se photodetectors under time evolution after light excitation. Moreover, the Bi2 O2 Se bolometer shows a high temperature coefficient of resistance (-1.6% K-1 ), high bolometric coefficient (-31 nA K-1 ), and high bolometric responsivity (>320 A W-1 ). These findings offer a new approach to develop bolometric photodetectors based on Bi2 O2 Se layered materials.

In-plane anisotropy in twisted bilayer graphene probed by Raman spectroscopy.

Monolayer graphene has high symmetrical crystal structure and exhibits in-plane isotropic physical properties. However, twisted bilayer graphene (tBLG) is expected to differ physically, due to the broken symmetry introduced by the interlayer coupling between adjacent graphene layers. This symmetry breaking is usually accompanied by in-plane anisotropy in their electrical, optical and thermal properties. However, the existence of in-plane anisotropy in tBLG has remained evasive until now. Here, an unambiguous identification of the in-plane anisotropy in tBLG is established by angle-resolved polarized Raman spectroscopy. It was found that the double-resonant two-dimensional band is anisotropic. The degree of in-plane anisotropy is found to be dependent on the misorientation angles, which is two- and four-fold for tBLG with misorientation angles of 15° and 20°, respectively. This finding adds a new dimension to the properties of graphene, which opens a possibility to the development of graphene-based angle-resolved photonics and electronics.

Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires and Two-Dimensional MoTe2 Ultrathin Films Heterostructures.

Controllable synthesizing of one-dimensional-two-dimensional (1D-2D) heterostructures and tuning their atomic and electronic structures is nowadays of particular interest due to the extraordinary properties and potential applications. Here, we demonstrate the temperature-induced phase-controlled growth of 1D Mo6Te6-2D MoTe2 heterostructures via molecular beam epitaxy. In situ scanning tunneling microscopy study shows 2D ultrathin films are synthesized at low temperature range, while 1D nanowires gradually arise and dominate as temperature increasing. X-ray photoelectron spectroscopy confirms the good stoichiometry and scanning tunneling spectroscopy reveals the semimetallic property of grown Mo6Te6 nanowires. Through in situ annealing, a phase transition from 2D MoTe2 to 1D Mo6Te6 is induced, thus forming a semimetal-semiconductor junction in atomic level. An upward band bending of 2H-MoTe2 is caused by lateral hole injection from Mo6Te6. The work suggests a new route to synthesize 1D semimetallic transition metal chalcogenide nanowires, which could serve as ultrasmall conducting building blocks and enable band engineering in future 1D-2D heterostructure devices.

Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure.

van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.

[Study on Raman spectra of multi-walled carbon nanotubes with different parameters].

In order to study the influencing factors on Raman spectroscopy, we research a series of comparative Raman spectroscopy of multi-walled carbon nanotubes (MWCNT) with different tube diameter and length. The results suggest that the G peak and D peak of MWCNT are all red-shifted as compared to that of polycrystalline graphite; In the same conditions, the peak intensity (G peak and D peak) is directly proportional to the diameter of the MWCNT, and inversely proportional to the length of the MWCNT; G peak frequency shift is closely related to the MWCNT diameter and length, which are inversely proportional to the diameter (with identical results of the single-walled carbon nanotube radial breathing modes) and direct proportional to the length. While, the influences of the diameter and length on D peak frequency shift are weak, and future analysis for the reason of this kind of phenomenon is as follows. Subsequently, we investigated the relation between D peak frequency shift and MWCNT aspect ratio, the relationship between G peak frequency shift and aspect ratio is nearly linear increase. Using the same analysis method, we plotted the different graphs of G peak and D peak intensity vs the aspect ratio of MWCNT, respectively. As the expected, the linear degression relation are existent in the two relationships.

Depletion Strategies for Crystallized Layers of Two-Dimensional Nanosheets to Enhance Lithium-Ion Conductivity in Polymer Nanocomposites.

The assembly of long-range aligned structures of two-dimensional nanosheets (2DNSs) in polymer nanocomposites (PNCs) is in urgent need for the design of nanoelectronics and lightweight energy-storage materials of high conductivity for electricity or heat. These 2DNS are thin and exhibit thermal fluctuations, leading to an intricate interplay with polymers in which entropic effects can be exploited to facilitate a range of different assemblies. In molecular dynamics simulations of experimentally studied 2DNSs, we show that the layer-forming crystallization of 2DNSs is programmable by regulating the strengths and ranges of polymer-induced entropic depletion attractions between pairs of 2DNSs, as well as between single 2DNSs and a substrate surface, by exclusively tuning the temperature and size of the 2DNS. Enhancing the temperature supports the 2DNS-substrate depletion rather than crystallization of 2DNSs in the bulk, leading to crystallized layers of 2DNSs on the substrate surfaces. On the other hand, the interaction range of the 2DNS-2DNS depletion attraction extends further than the 2DNS-substrate attraction whenever the 2DNS size is well above the correlation length of the polymers, which results in a nonmonotonic dependence of the crystallization layer on the 2DNS size. It is demonstrated that the depletion-tuned crystallization layers of 2DNSs contribute to a conductive channel in which individual lithium ions (Li ions) migrate efficiently through the PNCs. This work provides statistical and dynamical insights into the balance between the 2DNS-2DNS and 2DNS-substrate depletion interactions in polymer-2DNS composites and highlights the possibilities to exploit depletion strategies in order to engineer crystallization processes of 2DNSs and thus to control electrical conductivity.

Ship-in-bottle synthesis of gold nanoparticles in amino-functionalized mesoporous silica thin films.

Amino-functionalized mesoporous silica thin films with high content of -NH2 moieties have been directly dip-coated on silicon wafers by co-condensation of tetraethoxysilane and 3-aminopropyltriethoxysilane in the presence of C16H33(OCH2CH2)10OH under acidic condition. X-ray diffraction patterns show that it is a good strategy to remove the surfactant by ethanol extraction rather than calcinations. Using the amino-functionalized mesoporous silica thin films as templates, nanoparticles arrays of gold are formed well within the mesostructures. The isolated gold nanoparticles with size of 2.4-3.9 nm are released by dissolving the silica frameworks with the help of an ethanol solution of HF, showing that the gold nanoparticles have uniform size and spherical appearance. The Ultraviolet-visible spectra show that as the size of gold nanoparticles increase when the dipping time extended, the surface plasmon resonance absorption peak shifts to a longer wavelength.

Charge transport property of mesoporous silica thin films incorporated with gold nanoparticles.

Highly ordered mesoporous silica thin films (MSTFs) were spin-coated on Si wafers by self-assemble of water-soluble nanocrystal micelles with soluble silica. The gold nanoparticles (GNPs) were assembled within the silica matrix. Based on this nanocomposite, nanoscale electrodes were fabricated to form single electron devices. Two main transport behaviors of resemblant Coulomb blockage were demonstrated in the current-voltage measurements. The Coulomb island size calculated from the current-voltage data was close to the real size of the GNPs. The random loading positions of GNPs in the MSTF, which lead to uncertain tunneling resistances, might be response for the conductive differences.

Fabrication and characterization of single electron tunneling device on Au/SiO2 nanocomposite films.

Amino-functionalized mesoporous silica thin films (AF-MSTFs) have been directly dip-coated on silica wafers by co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) in the presence of Brij56 (C16H33(OCH2CH2)10OH) under acidic condition. Using the AF-MSTFs as templates, gold nanoparticles (GNPs) are well formed within the mesopores. The transmission electron microscope images and X-ray diffraction patterns demonstrate that the MSTFs are consistent with well ordered mesostructures and the GNPs are well crystallized in the templates. A single electron tunneling device based on self-assembled GNPs of the Au/SiO2 nanocomposite film has been fabricated by using nanolithographic definitions. The device shows Coulomb stair-cases at 77 K. The data are interpreted as single electron tunneling through the GNP which served as a Coulomb island between the source and drain electrodes. The charging energy is estimated to be much larger than the thermal energy at 77 K. The Coulomb island size calculated from the experimental data is -3.8 nm, which is consistent with the size of the GNPs self-assembled in the MSTFs.

Photocurrent imaging of CdS/Al interfaces based on microscopic analysis.

We investigate the lateral variations of photocurrent on CdS/Al interfaces, with a combination of a semiconductor characterization system and scanning near-field optical microscopy, in which the near-field probe is used to locally induce photocurrent on the CdS/Al interfaces with high spatial resolution. By analyzing the spatially resolved photoresponse, we find that the resolution is worsened in the photocurrent images by the lateral diffusion of the photoexcited electrons and that the photoelectric properties of the CdS/Al interfaces are strongly affected by the bias voltage. Furthermore, in a complementary experiment, we also demonstrate that the photocurrent measurements can reveal structures that are not present in the case of shear-force data. The analysis demonstrates the band structure and microscopic mechanism of CdS/Al heterostructures, which provide an effective approach for developing CdS-based photoelectronic devices.

Role of Thermal Process on Self-Assembled Structures of 4'-([2,2':6',2''-Terpyridin]-4'-Yl)-[1,1'-Biphenyl]-4-Carboxylic Acid on Au(III).

The role of dynamic processes on self-assembled structures of 4'-([2,2':6', 2''-terpyridin]-4'-yl)-[1,1'-biphenyl]-4-carboxylic acid (l) molecules on Au(III) has been studied by scanning tunneling microscopy. The as-deposited monolayer is closed-packed and periodic in a short-range due to dipole forces. A thermal annealing process at 110 degrees drives such disordered monolayer into ordered chain-like structures, determined by the combination of the dipole forces and hydrogen bonding. Further annealing at 130 degrees turns the whole monolayer into a bowknot-like structure in which hydrogen bonding plays the dominant role in the formation of assembled structures. Such dependence of an assembled structure on the process demonstrates that an assembled structure can be regulated and controlled not only by the molecular structure but also by the thermal process to form the assembled structure.

Impact of Sapphire Step Height on the Growth of Monolayer Molybdenum Disulfide.

Although the synthesis of molybdenum disulfide (MoS2) on sapphire has made a lot of progress, how the substrate surface affects the growth still needs to be further studied. Herein, the impact of the sapphire step height on the growth of monolayer MoS2 through chemical vapor deposition (CVD) is studied. The results show that MoS2 exhibits a highly oriented triangular grain on a low-step (0.44-1.54 nm) substrate but nanoribbons with a consistent orientation on a high-step (1.98-3.30 nm) substrate. Triangular grains exhibit cross-step growth, with one edge parallel to the step edge, while nanoribbons do not cross steps and possess the same orientation as the step. Scanning electron microscopy (SEM) reveals that nanoribbons are formed by splicing multiple grains, and the consistency of the orientation of these grains is demonstrated with a transmission electron microscope (TEM) and second-harmonic generation (SHG). Furthermore, our CP2K calculations, conducted using the generalized gradient approximation and the Perdew-Burke-Ernzerhof (PBE) functional with D3 (BJ) correction, show that MoS2 domains prefer to nucleate at higher steps, while climbing across a higher step is more difficult. This work not only sheds light on the growth mechanism of monolayer MoS2 but also promotes its applications in electrical, optical, and energy-related devices.

Symmetry-Engineering-Induced In-Plane Polarization Enhancement in Ta2NiS5/CrOCl van der Waals Heterostructure.

Van der Waals (vdW) interfaces can be formed via layer stacking regardless of the lattice constant or symmetry of the individual building blocks. Herein, we constructed a vdW interface of layered Ta2NiS5 and CrOCl, which exhibited remarkably enhanced in-plane anisotropy via polarized Raman spectroscopy and electrical transport measurements. Compared with pristine Ta2NiS5, the anisotropy ratio of the Raman intensities for the B2g, 2Ag, and 3Ag modes increased in the heterostructure. More importantly, the anisotropy ratios of conductivity and mobility in the heterostructure increased by one order of magnitude. Specifically speaking, the conductivity ratio changed from ~2.1 (Ta2NiS5) to ~15 (Ta2NiS5/CrOCl), while the mobility ratio changed from ~2.7 (Ta2NiS5) to ~32 (Ta2NiS5/CrOCl). Such prominent enhancement may be attributed to the symmetry reduction caused by lattice mismatch at the heterostructure interface and the introduction of strain into the Ta2NiS5. Our research provides a new perspective for enhancing artificial anisotropy physics and offers feasible guidance for future functionalized electronic devices.

Improved Thermal Anisotropy of Multi-Layer Tungsten Telluride on Silicon Substrate.

WTe2, a low-symmetry transition metal dichalcogenide, has broad prospects in functional device applications due to its excellent physical properties. When WTe2 flake is integrated into practical device structures, its anisotropic thermal transport could be affected greatly by the substrate, which matters a lot to the energy efficiency and functional performance of the device. To investigate the effect of SiO2/Si substrate, we carried out a comparative Raman thermometry study on a 50 nm-thick supported WTe2 flake (with κzigzag = 62.17 W·m-1·K-1 and κarmchair = 32.93 W·m-1·K-1), and a suspended WTe2 flake of similar thickness (with κzigzag = 4.45 W·m-1·K-1, κarmchair = 4.10 W·m-1·K-1). The results show that the thermal anisotropy ratio of supported WTe2 flake (κzigzag/κarmchair ≈ 1.89) is about 1.7 times that of suspended WTe2 flake (κzigzag/κarmchair ≈ 1.09). Based on the low symmetry nature of the WTe2 structure, it is speculated that the factors contributing to thermal conductivity (mechanical properties and anisotropic low-frequency phonons) may have affected the thermal conductivity of WTe2 flake in an uneven manner when supported on a substrate. Our findings could contribute to the 2D anisotropy physics and thermal transport study of functional devices based on WTe2 and other low-symmetry materials, which helps solve the heat dissipation problem and optimize thermal/thermoelectric performance for practical electronic devices.

Effect of Nanoscale in Situ Interface Welding on the Macroscale Thermal Conductivity of Insulating Epoxy Composites: A Multiscale Simulation Investigation.

Insulating thermally conductive polymer composites are in great demand in integrated-circuit packages, for efficient heat dissipation and to alleviative short-circuit risk. Herein, the continuous oriented hexagonal boron nitride (h-BN) frameworks (o-BN@SiC) were prepared via self-assembly and in situ chemical vapor infiltration (CVI) interface welding. The insulating o-BN@SiC/epoxy (o-BN@SiC/EP) composites exhibited enhanced thermal conductivity benefited from the CVI-SiC-welded BN-BN interface. Further, multiscale simulation, combining first-principles calculation, Monte Carlo simulation, and finite-element simulation, was performed to quantitatively reveal the effect of the welded BN-BN interface on the heat transfer of o-BN@SiC/EP composites. Phonon transmission in solders and phonon-phonon coupling of filler-solder interfaces enhanced the interfacial heat transfer between adjacent h-BN microplatelets, and the interfacial thermal resistance of the dominant BN-BN interface was decreased to only 3.83 nK·m2/W from 400 nK·m2/W, plunging by over 99%. This highly weakened interfacial thermal resistance greatly improved the heat transfer along thermal pathways and resulted in a 26% thermal conductivity enhancement of o-BN@SiC/EP composites, compared with physically contacted oriented h-BN/EP composites, at 15 vol % h-BN. This systematic multiscale simulation broke through the barrier of revealing the heat transfer mechanism of polymer composites from the nanoscale to the macroscale, which provided rational cognition about the effect of the interfacial thermal resistance between fillers on the thermal conductivity of polymer composites.

Controlling anisotropic thermal properties of graphene aerogel by compressive strain.

Materials with adjustable wide-ranging thermal conductivity are desired to tackle the problem of thermal management for electronic devices operating in an extended range of temperature. In this study, graphene aerogels (GAs) are fabricated and transformed from thermal insulators to thermal conductors by high-temperature annealing. The highest through-plane and in-plane thermal conductivity of annealed GA reaches 3.3 and 96 W/m·K, respectively, under 95% compressive strain. Using the annealed GA as thermal interface material leads to superior performance than commercially available products that have higher through-plane thermal conductivity in dissipating heat for high-power electronic devices (e.g., LED lamp). Furthermore, due to excellent elasticity, the thermal resistance of annealed GAs can be reversibly tuned about six-fold by compressive strain. This paves a novel venue in designing thermal management system for devices, which not only need excellent heat dissipation but also good thermal insulation at various operating environments.

Energy Dissipation and Electrical Breakdown in Multilayer PtSe2 Electronics.

Investigating the energy dissipation in micro- and nanoscale is fundamental to improve the performance and reliability of two-dimensional (2D) electronics. Recently, 2D platinum selenide (PtSe2) has drawn extensive attention in developing next-generation functional devices due to its distinctive fusion of versatile properties. Toward practical applications of PtSe2 devices, it is essential to understand the interfacial thermal properties between PtSe2 and its substrate. Among them, the thermal boundary conductance (TBC) has played a critical role for out-of-plane heat dissipation of PtSe2 devices. Here, we identify the energy dissipation behavior of multilayer PtSe2 devices and extract the actual TBC value of the PtSe2/SiO2 interface by Raman thermometry with electrical bias. The obtained TBC value is about 8.6 MW m-2 K-1, and it belongs to the low end of as-known solid-solid interfaces, suggesting possible applications regarding thermoelectric devices or others reliant on a large temperature gradient. Furthermore, the maximum current density of the PtSe2 device determines its threshold power, which is crucial for improving device design and guiding future applications. Therefore, we explore the electrical breakdown profile of the multilayer PtSe2 device, revealing the breakdown current density of 17.7 MA cm-2 and threshold power density of 0.2 MW cm-2, which are larger than typical values for commonly used aluminum and copper. These results provide key insights into the energy dissipation of PtSe2 devices and make PtSe2 an excellent candidate for thermal confinement applications and nanometer-thin interconnects, which will benefit the development of energy-efficient functional 2D devices.

Free-standing graphene aerogel with improved through-plane thermal conductivity after being annealed at high temperature.

Both high through-plane thermal conductivity and low elastic modulus can reduce thermal interface resistance, which is important for thermal interface materials. The internal porous structure of graphene aerogel (GA) makes it to have a low elastic modulus, which results in its good compressibility. Also, the network structure of GA provides thermal conducting paths, which improve the through-plane thermal conductivity of GA. Annealing GA at 3000 °C helps to remove oxygen-containing functional groups and reduces defects. This greatly improves its crystallinity, which further leads to the improvement of its through-plane thermal conductivity and it has a low modulus of 1.37Mpa. The through-plane thermal conductivity of GA annealed at 3000 °C (GA-3000) was improved as the pressure increased and got to 2.93 W/ m K at a pressure of 1.13 MPa, which is 30 times higher than other graphene-based thermal interface materials (TIMs). These discoveries offer a novel approach for preparing excellent TIMs.

Chemical Vapor Deposition of Uniform and Large-Domain Molybdenum Disulfide Crystals on Glass/Al2O3 Substrates.

Two-dimensional molybdenum disulfide (MoS2) has attracted significant attention for next-generation electronics, flexible devices, and optical applications. Chemical vapor deposition is the most promising route for the production of large-scale, high-quality MoS2 films. Recently, the chemical vapor deposition of MoS2 films on soda-lime glass has attracted great attention due to its low cost, fast growth, and large domain size. Typically, a piece of Mo foil or graphite needs to be used as a buffer layer between the glass substrates and the CVD system to prevent the glass substrates from being fragmented. In this study, a novel method was developed for synthesizing MoS2 on glass substrates. Inert Al2O3 was used as the buffer layer and high-quality, uniform, triangular monolayer MoS2 crystals with domain sizes larger than 400 µm were obtained. To demonstrate the advantages of glass/Al2O3 substrates, a direct comparison of CVD MoS2 on glass/Mo and glass/Al2O3 substrates was performed. When Mo foil was used as the buffer layer, serried small bilayer islands and bright core centers could be observed on the MoS2 domains at the center and edges of glass substrates. As a control, uniform MoS2 crystals were obtained when Al2O3 was used as the buffer layer, both at the center and the edge of glass substrates. Raman and PL spectra were further characterized to show the merit of glass/Al2O3 substrates. In addition, the thickness of MoS2 domains was confirmed by an atomic force microscope and the uniformity of MoS2 domains was verified by Raman mapping. This work provides a novel method for CVD MoS2 growth on soda-lime glass and is helpful in realizing commercial applications of MoS2.