Postdoctoral Associate, Massachusetts Institute of Technology (MIT)
Dielectric performance of large-area hexagonal boron nitride
Hexagonal boron nitride (h-BN) is a two dimensional (2D) layered insulator with exceptional properties including high in-plane mechanical strength, large thermal conductivity and high chemical stability. To date, it remains challenging to obtain large area and high quality hBN, and most of the research efforts still relying small h-BN flakes (micrometer-scale) exfoliated from bulk crystal with undefined thickness. In this talk, we report a chemical vapor deposition (CVD) approach to obtain large-area (centimeter-scale) h-BN films with controlled thickness. We demonstrate that h-BN films can be grown on a Pt substrate using borazine precursor and its thickness can be controlled by tuning the crystallographic orientations of the Pt substrate. Specifically, Pt (101) grain yields thicker h-BN films, while Pt(001) and Pt (111) grains result in thinner h-BN films. Our synthesized h-BN films on Pt substrate exhibit good properties as confirmed by Raman, AFM, TEM and EELS analyses. In addition, we compare the electrical reliability of multilayer CVD-grown h-BN films as a function of its thickness. For device level characterization, we have fabricated and tested more than 200 Au/Ti/h-BN/Pt metal-insulator-metal (MIM) devices with an area size of 2500 um2. Through current-time (I-t) characterization under 100 mV of constant voltage stressing (CVS), we observed that the leakage current density of thicker h-BN films is about 3 orders of magnitude smaller than that of thinner h-BN films, although both films were grown on the same Pt substrate under the same conditions. More interestingly, the thicker h-BN films show step by step current increment under CVS (e.g. 200 mV and 300 mV) for 300 seconds, while that of thinner h-BN films remain unchanged. In addition, current-voltage (I-V) studies clearly indicate that thicker h-BN shows much slower degradation compared to thinner h-BN after stressing at different constant voltages. In brief, the crystalline nature of the growth substrate plays a significant role in the electrical reliability and robustness of the CVD-grown large-area h-BN.
Abstract: Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetector owing to its tunable band gap and high absorption coefficient. However, controlled synthesis of high quality, large area monolayer molybdenum disulfide (MoS2) is still a challenge in practical application. In this work, we report a gold foil assistant chemical vapor deposition (CVD) method of large size (>400 μm) single crystal MoS2 film on silicon dioxide (SiO2) substrate. The influence of Au foil in enlarging the size of single crystal MoS2 were investigated systemically using thermal simulation in Ansys workbench 16.0, including thermal conductivity, temperature difference and thermal relaxation time of the interface of SiO2 substrate and Au foil, which indicated that Au foil could increase the temperature of the SiO2 substrate rapidly and decrease the temperature difference between the oven and substrate. At last, the property of the monolayer MoS2 film was further confirmed by the back-gated field effect transistors (FETs), a high photo-response of 15.6 A/W and a fast photo-response time of 100 ms was obtained. The growth techniques described in this study could be benefit for the development of other atomically thin two-dimensional transition metal dichalcogenides (TMD) materials.
Pub.: 23 May '17, Pinned: 30 Aug '17
Abstract: Polymer residue plays an important role in the performance of 2D heterostructured materials. Herein, we study the effect of polymer residual impurities on the electrical properties of graphene-boron nitride planar heterostructures. Large-area graphene (Gr) and hexagonal boron nitride (h-BN) monolayers were synthesized using chemical vapor deposition (CVD) techniques. Atomic van-der-Waals heterostructure layers based on varied configurations of Gr and h-BN layers were assembled. The average interlayer resistance of the heterojunctions over a 1 cm2 area for several planar heterostructure configurations was assessed by impedance spectroscopy and modeled by equivalent electrical circuits. Conductive AFM measurements showed that the presence of polymer residues on the surface of the Gr and h-BN monolayers resulted in significant resistance deviations over nanoscale regions.
Pub.: 31 May '17, Pinned: 30 Aug '17
Abstract: Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS2 exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe2 bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe2 bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe2 monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe2 bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe2 bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe2 for which the device performance becomes degraded above a strain of 0.63%.
Pub.: 07 Jun '17, Pinned: 30 Aug '17
Abstract: Molybdenum disulfide (MoS<sub>2</sub>), as one of the atomically thin two-dimensional transition metal dichalcogenides has novel layer-dependent optical and electronic properties, which make it competitive for potential applications in optoelectronics. Here, we report chemical vapor deposition (CVD) growth of vertically-standing and planar spiral MoS<sub>2</sub> nanosheets. These vertical spiral MoS<sub>2</sub> nanosheets are formed by the compression between planar spiral MoS2 in a close proximity. Both structures have a polytype 3R stacking with broken inversion symmetry, exhibiting strong second and third harmonic generations.
Pub.: 09 Jun '17, Pinned: 30 Aug '17
Abstract: Two-dimensional materials such as graphene offer fundamentally transformative opportunities in membrane separations and as impermeable barriers, but the lack of facile methods to assess and control its 'impermeability' critically limits progress. Here we show that a simple etch of the growth catalyst (Cu) through defects in monolayer graphene synthesized by chemical vapor deposition (CVD) can be used to effectively assess graphene quality for membrane/barrier applications. Using feedback from the method to tune synthesis, we realize graphene with nearly no nanometer-scale defects as assessed by diffusion measurements, in contrast to commercially available graphene that is largely optimized for electronic applications. Interestingly, we observe clear evidence of leakage through larger defects associated with wrinkles in graphene, which are selectively sealed to realize centimeter-scale atomically thin barriers exhibiting <2% mass transport compared to the graphene support. Our work provides a facile method to assess and control the 'impermeability' of graphene and shows that future work should be directed towards the control of leakage associated with wrinkles.
Pub.: 13 Jun '17, Pinned: 30 Aug '17
Abstract: Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe2 is completely recovered if the material is sandwiched in MoS2/MoSe2/MoS2 trilayer van der Waals heterostructures. We show by means of density-functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS2 layers are donated to heal Se vacancy defects in the middle MoSe2 layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe2 without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality.
Pub.: 13 Jun '17, Pinned: 30 Aug '17
Abstract: Recently, a contamination-free dry transfer method for graphene grown by chemical vapor deposition (CVD) has been reported that allows to directly pick-up graphene from the copper growth substrate using a flake of hexagonal boron nitride (hBN), resulting in ultrahigh charge carrier mobility and low overall doping. Here, we report that not only hBN, but also flakes of molybdenum disulfide (MoS2) can be used to dry transfer graphene. This, on one hand, allows for the fabrication of complex van-der-Waals heterostructures using CVD graphene combined with different two-dimensional materials and, on the other hand, can be a route toward a scalable dry transfer of CVD graphene. The resulting heterostructures are studied using low temperature transport measurements revealing a strong charge carrier density dependence of the carrier mobilities (up to values of 12,000 cm2/(Vs)) and the residual charge carrier density fluctuations near the charge neutrality point when changing the carrier density in the MoS2 by applying a top gate voltage.
Pub.: 14 Jun '17, Pinned: 30 Aug '17
Abstract: In the last decade, few-layer boron nitride nanosheets and monolayer BN nanosheet (BNNS) have gained much attention due to their unique physical and chemical properties. To date, BNNS can be produced by micromechanical cleavage of BN crystal, unzipping BN nanotubes, chemical vapor deposition (CVD), and solution processed exfoliation from bulk BN powder. Due to the low cost and abundance of bulk BN powder and its simple processing for potentially scalable production, great efforts have been devoted toward solution processed exfoliation. In this feature article, recent significant advances in solution processed synthesis of BNNS are summarized. In particular, the solvent choice for one-step BN exfoliation is highlighted. Multi-dimensional assemblies consisting of BNNS are discussed, such as BN fiber, BN paper, and BN aerogel. The emerging applications of BNNS in different fields are then focused on, especially in barrier materials, dielectrics, catalysts, and thermal management.
Pub.: 19 Jun '17, Pinned: 30 Aug '17
Abstract: This work presents the synthesis of carbon nanotubes–nanoporous anodic alumina composite membranes (CNTs–NAAMs) with controllable geometric features by a template-assisted catalyst-free chemical vapor deposition (CVD) approach using a mixture of toluene and ethanol as a carbon precursor. NAAMs templates were prepared by anodization of aluminum substrates in different electrolytes containing sulfuric, oxalic, and phosphoric acids with the aim of establishing the template effect on the CNTs growth. The deposition time during the CVD process was systematically modified to determine the formation mechanism of CNTs inside the pores of NAAMs without using metal catalysts. The structural features, chemical composition, and graphitic structures of the resulting CNTs–NAAMs composites were characterized by different techniques to provide a comprehensive understanding of the effect of the template on the formation of these carbon-based nanostructures. CNTs–NAAMs with inner pore diameters ranging from 15 to 180 nm were used. Our results reveal that the electrolyte type used to prepare NAAMs and the deposition time during the CVD process have a direct impact on the structural, chemical, and graphitic structural features of CNTs–NAAMs. The molecular transport properties of CNTs–NAAMs composite membranes featuring different geometries and chemical compositions were evaluated via the diffusion process of Rose Bengal, a dye model molecule. The obtained results show that the diffusional flux of the dye molecules can be controlled by tuning the inner pore diameter of CNTs deposited inside NAAMs, and the smaller the diameter of the nanotubes the faster the transport of dye molecules is. Our results provide novel insights into the fabrication of different CNTs composite membranes, establishing for the first time the influence of three common types of NAAMs templates on the properties of the resulting CNTs composite membranes. Our study enables the precise engineering of advanced CNTs composite membranes with controlled physical and chemical properties suitable for specific applications.
Pub.: 01 Jun '17, Pinned: 30 Aug '17
Abstract: The development trend of modern electronics and optoelectronics is towards continuous high integration and miniaturization. Using vertical configurations with three-dimensional geometry, it is easy to establish a higher integration density than the traditional planar one, and thus, this technology shows great promise for designing the next-generation electronics/optoelectronic devices. Two-dimensional (2D) layered metal dichalcogenides (2D-LMDs) are important building blocks for electronic/optoelectronic devices, where they are usually grown in parallel to the substrates during chemical vapor deposition (CVD), and consequently they are solely exploited to fabricate lateral structure devices with planar geometry. In this research, for the first time the vertical growth of free standing 2D layered nanosheets of hexagonal tin disulfide (SnS2) on a flat substrate was realized using a modified CVD method. Furthermore, it was successfully demonstrated, at the first attempt, that a type of non-planar vertical photodetector could be fabricated using free standing SnS2 nanosheets and this detector showed promise for photodetection applications. This work prepares the way for the growth of monodisperse vertical 2D-LMD nanosheets on flat substrates, and expands their use from conventional lateral structure devices to non-planar vertical electronic/optoelectronic devices.
Pub.: 27 Jun '17, Pinned: 30 Aug '17
Abstract: We report the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS$_2$) - silicon dioxide (SiO$_2$) one-dimensional, free-standing photonic-crystal cavities capable of enhancement of the MoS$_2$ photoluminescence at the narrow cavity resonance. We demonstrate continuous tunability of the cavity resonance wavelength across the entire emission band of MoS$_2$ simply by variation of the photonic crystal periodicity. Device fabrication started by substrate-scale growth of MoS$_2$ using chemical vapor deposition (CVD) on non-birefringent thermal oxide on a silicon wafer; it was followed by lithographic fabrication of a photon crystal nanocavity array on the same substrate at more than 50% yield of functional devices. Our cavities exhibit three dominant modes with measured linewidths less than 0.2 nm, corresponding to quality factors exceeding 4000. All experimental findings are found to be in excellent agreement with finite difference time domain simulations.
Pub.: 28 Jun '17, Pinned: 30 Aug '17
Abstract: Dialysis is a ubiquitous separation process in biochemical processing and biological research. State-of-the-art dialysis membranes comprise a relatively thick polymer layer with tortuous pores, and suffer from low rates of diffusion leading to extremely long process times (often several days) and poor selectivity, especially in the 0-1000 Da molecular weight cut-off range. Here, the fabrication of large-area (cm(2) ) nanoporous atomically thin membranes (NATMs) is reported, by transferring graphene synthesized using scalable chemical vapor deposition (CVD) to polycarbonate track-etched supports. After sealing defects introduced during transfer/handling by interfacial polymerization, a facile oxygen-plasma etch is used to create size-selective pores (≤1 nm) in the CVD graphene. Size-selective separation and desalting of small model molecules (≈200-1355 Da) and proteins (≈14 000 Da) are demonstrated, with ≈1-2 orders of magnitude increase in permeance compared to state-of-the-art commercial membranes. Rapid diffusion and size-selectivity in NATMs offers transformative opportunities in purification of drugs, removal of residual reactants, biochemical analytics, medical diagnostics, therapeutics, and nano-bio separations.
Pub.: 29 Jun '17, Pinned: 30 Aug '17
Abstract: Transition metal dichalcogenides (TMDC) like MoS2 are promising candidates for next-generation electric and optoelectronic devices. These TMDC monolayers are typically synthesized by chemical vapor deposition (CVD). However, despite significant amount of empirical work on this CVD growth of mono-layered crystals, neither experiment nor theory has been able to decipher mechanisms of selection rules for different growth scenarios, or make predictions of optimized environmental parameters and growth factors. Here, we present an atomic-scale mechanistic analysis of the initial sulfidation process on MoO3 surfaces using first principles-informed ReaxFF reactive molecular dynamics (RMD) simulations. We identify a three-step reaction process associated with synthesis of the MoS2 samples from MoO3 and S2 precursors: O2 evolution and self-reduction of the MoO3 surface; SO/SO2 formation and S2-assisted reduction; and sulfidation of the reduced surface and Mo-S bond formation. These atomic processes occurring during early-stage MoS2 synthesis, which are consistent with experimental observations and existing theoretical literature, provide valuable input for guided rational synthesis of MoS2 and other TMDC crystals by the CVD process.
Pub.: 04 Jul '17, Pinned: 30 Aug '17
Abstract: The monolayer semiconductors of molybdenum disulfide (MoS2) crystals have drawn tremendous attentions due to their extraordinary electronic and optical properties. Uniform and high-quality crystalline MoS2 monolayer is greatly needed in fundamental studies and practical applications. Three-point star to six-point star MoS2 nanosheets are readily synthesized in controlled manner using chemical vapor deposition (CVD) method. A possible coalescent model is proposed to study the evolution of the six-point star MoS2 domain. A comparative study of the field effect transistors (FETs) are performed to disclose the negative effect of grain boundaries (GBs) on the transport properties based on the six-point star MoS2.
Pub.: 04 Jul '17, Pinned: 30 Aug '17
Abstract: Chemical vapor deposition (CVD) is a powerful technique to produce graphene for large scale applications. Polymer-assisted wet transfer is commonly used to move the graphene onto silicon substrates but the resulting devices tend to exhibit p-doping which decreases the device quality and reproducibility. In an effort to better understand the origin of this effect we coated graphene with n-methyl-2-pyrrolidone (NMP) and hexamethyldisilazane (HMDS) molecules that exhibit negligible charge transfer to graphene but bind more strongly to graphene than ambient adsorbents. Using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electrical transport measurements, and quantum mechanical computer simulations, we show that the molecules help in the removal of p-doping and our data indicates that the molecules do this by replacing ambient adsorbents (typically, O2 and water) on the graphene surface. This very simple method of improving the electronic properties of CVD graphene by passivating its surface with common solvent molecules will accelerate the development of CVD graphene based devices.
Pub.: 05 Jul '17, Pinned: 30 Aug '17
Abstract: Recently there have been many research breakthroughs in two-dimensional (2D) materials including graphene, boron nitride (h-BN), black phosphors (BPs), and transition-metal dichalcogenides (TMDCs). The unique electrical, optical, and thermal properties in 2D materials are associated with their strictly defined low dimensionalities. These materials provide a wide range of basic building blocks for next-generation electronics. The chemical vapor deposition (CVD) technique has shown great promise to generate high-quality TMDC layers with scalable size, controllable thickness, and excellent electronic properties suitable for both technological applications and fundamental sciences. The capability to precisely engineer 2D materials by chemical approaches has also given rise to fascinating new physics, which could lead to exciting new applications. In this Review, we introduce the latest development of TMDC synthesis by CVD approaches and provide further insight for the controllable and reliable synthesis of atomically thin TMDCs. Understanding of the vapor-phase growth mechanism of 2D TMDCs could benefit the formation of complicated heterostructures and novel artificial 2D lattices.
Pub.: 07 Jul '17, Pinned: 30 Aug '17
Abstract: Synthesis of monolayer transition metal dichalcogenides (TMDs) via chemical vapor deposition (CVD) relies on several factors such as precursor, promoter, substrate, and surface treatment of substrate. Among them, the use of promoter is crucial for obtaining uniform and large-area monolayer TMDs. Although promoters have been speculated to enhance adhesion of precursors to the substrate, their precise role in the growth mechanism has rarely been discussed. Here, we report the role of alkali metal promoter in growing monolayer TMDs. The growth occurred via the formation of sodium metal oxides which prevent the evaporation of metal precursor. Furthermore, the silicon oxide substrate helped to decrease the Gibbs free energy by forming sodium silicon oxide compounds. The resulting sodium metal oxide was anchored within such concavities created by corrosion of silicon oxide. Consequently, the wettability of the precursors to silicon oxide was improved, leading to enhance lateral growth of monolayer TMDs.
Pub.: 08 Jul '17, Pinned: 30 Aug '17
Abstract: Despite much interest in applications of two-dimensional (2D) fabrics such as MoS2, to date most studies have focused on individual or few devices. Here we examine the variability of hundreds of transistors fabricated from large-area monolayer MoS2 synthesized by chemical vapor deposition (CVD). Ultraclean fabrication yields very low surface roughness of ~3 Å and surprisingly low variability of key device parameters, considering the atomically thin nature of the material. Across continuous films >1 cm(2) the field-effect mobility averages 34.2 cm(2)/V/s with coefficient of variation 0.1; threshold voltage variation corresponds to < 8×10(11) cm(-2) variation in fixed charge; and very low hysteresis suggests ~10(11) cm(-2) density of charge traps. The electrical properties of CVD-grown monolayer MoS2 are remarkably immune to the presence of bilayer regions, which cause only small conduction band offsets (~55 meV) measured by scanning Kelvin probe microscopy, an order of magnitude lower than energy variations in Si films of comparable thickness. The data are used as inputs to Monte Carlo circuit simulations to understand the effects of 2D variability on standard cell variation. These advances address key missing steps required to scale 2D semiconductors into functional systems.
Pub.: 12 Jul '17, Pinned: 30 Aug '17
Abstract: The authors propose a novel type of ion-selective membranes, which combine the advantages of ceramic nanofibrous media with good electrical conductivity. The membranes are produced from Nafen alumina nanofibers (diameter around 10 nm) by filtration of nanofiber suspension through a porous support followed by drying and sintering. Electrical conductivity is achieved by depositing a thin carbon layer on the nanofibers by chemical vapor deposition (CVD). Raman and FTIR spectroscopy, X-ray fluorescence analysis, and TEM are used to confirm the carbon structure formation. The deposition of carbon leads to decreasing porosity (from 75 to 62%) and specific surface area (from 146 to 107 m2 g−1) of membranes, while the pore size distribution maximum shifts from 28 to 16 nm. Measurements of membrane potential in an electrochemical cell show that the carbon coated membranes acquire high ionic selectivity (transference numbers 0.94 for anion and 0.06 for cation in aqueous KCl). Fitting the membrane potential data by the Teorell–Meyer–Sievers model shows that the fixed membrane charge increases proportionally with increasing electrolyte concentration. The carbon coated membranes are ideally polarizable for applied voltages from −0.5 to +0.8 V. The potential applications of produced membranes include nano- and ultrafiltration, separation of charged species, and switchable ion-transport selectivity.
Pub.: 10 Jul '17, Pinned: 30 Aug '17
Abstract: The possibility of practical realization of a two-stage chemical vapor deposition (CVD) process using as an example of nanocomposite coating “molybdenum disulfide (filler)–silicon oxide (matrix)” is experimentally demonstrated for the first time. Deposition is performed in a two-zone vertical tubular quartz reactor. The formation of molybdenum disulfide nanoparticles by spray pyrolysis is carried out in the upper zone, and the lower zone is used for deposition of nanocomposite coatings by atmospheric pressure plasma-enhanced CVD. An aerosol of the ammonium thiomolybdate solution in dimethylformamide is used for the synthesis of molybdenum disulfide particles, and silicon dioxide coatings are deposited from tetraethoxysilane. The structure and the composition of nanocomposite coating deposited are studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, and Fourier transform infrared spectroscopy (FTIR) spectroscopy.
Pub.: 10 Jul '17, Pinned: 30 Aug '17
Abstract: Micrometer sized oxidation patterns were created in chemical vapor deposition (CVD) grown graphene through scanning probe lithography (SPL) and then subsequently reduced by irradiation using a focused x-ray beam. Throughout the process, the films were characterized by lateral force microscopy (LFM), micro-Raman (µ-RS) and micro-x-ray photoelectron spectroscopy (µ-XPS). Firstly, the density of grain boundaries was found to be crucial in determining the maximum possible oxygen coverage with SPL. Secondly, the dominate factor in SPL oxidation was found to be the bias voltage. At low voltages, only structural defects are formed on grain boundaries. Above a distinct threshold voltage, oxygen coverage increased rapidly, with the duration of applied voltage affecting the final oxygen coverage. Finally, we found that, independent of initial conditions, types of defects or the amount of SPL oxidation, the same set of empirical coupled rate equations describes the reduction dynamics with the limiting reduction step being C-C→C=C.
Pub.: 18 Jul '17, Pinned: 30 Aug '17
Abstract: Polymer dielectrics are the preferred materials of choice for power electronics and pulsed power applications. However, their relatively low operating temperatures significantly limit their uses in harsh-environment energy storage devices, e.g., automobile and aerospace power systems. Herein, hexagonal boron nitride (h-BN) films are prepared from chemical vapor deposition (CVD) and readily transferred onto polyetherimide (PEI) films. Greatly improved performance in terms of discharged energy density and charge-discharge efficiency is achieved in the PEI sandwiched with CVD-grown h-BN films at elevated temperatures when compared to neat PEI films and other high-temperature polymer and nanocomposite dielectrics. Notably, the h-BN-coated PEI films are capable of operating with >90% charge-discharge efficiencies and delivering high energy densities, i.e., 1.2 J cm(-3) , even at a temperature close to the glass transition temperature of polymer (i.e., 217 °C) where pristine PEI almost fails. Outstanding cyclability and dielectric stability over a straight 55 000 charge-discharge cycles are demonstrated in the h-BN-coated PEI at high temperatures. The work demonstrates a general and scalable pathway to enable the high-temperature capacitive energy applications of a wide range of engineering polymers and also offers an efficient method for the synthesis and transfer of 2D nanomaterials at the scale demanded for applications.
Pub.: 18 Jul '17, Pinned: 30 Aug '17
Abstract: Precise, scalable, defect engineering of 2D nanomaterials is acutely sought after in contemporary materials science. Here we present defect engineering in monolayer graphene and molybdenum disulfide (MoS$_2$) by irradiation with noble gas ions at 30 keV. Two ion species of vastly different mass were used in a gas field ion source microscope: helium (He$^+$) and neon (Ne$^+$). A detailed study of the introduced defect sizes and interdefect distance with escalating ion dose was performed using Raman spectroscopy. Expanding on existing models, it is found that the average defect size is considerably smaller for supported than freestanding graphene and that the rate of defect production is larger. It is concluded that secondary atoms from the substrate play a significant role in defect production, creating numerous smaller defects rather than those created by the primary ion beam. Furthermore, a model was also applied to supported MoS$_2$, another promising member of the 2D material family. Corrective factors for both ions were obtained for MoS$_2$, demonstrating their different behaviour and facilitating comparison with other irradiation conditions in literature.
Pub.: 27 Jul '17, Pinned: 30 Aug '17
Abstract: In the future nanocircuits based on two-dimensional (2D) materials, the ideal nonvolatile memory should be based on 2D multiferroic materials that can combine both efficient ferroelectric writing and ferromagnetic reading, which remains hitherto unreported. Here we show first-principles evidences that halogen-intercalated phosphorene bilayer can be multiferroic with most long-sought advantages: their "mobile" magnetism can be controlled by ferroelectric switching upon external electric field, exhibiting either "on" state with spin-selective and highly p-doped channels, or "off" state insulating for both spin and electron transport, which renders efficient electrical writing and magnetic reading; vertical polarization can be maintained against depolarizing field, rendering high-density data storage possible; moreover, all those functions in the halogenated regions can be directly integrated into a 2D phosphorene wafer, like n/p channels by doping in a silicon wafer. Such formation of multiferroics with vertical polarization robust against depolarizing field can be attributed to the unique properties of covalent-bonded ferroelectrics distinct from ionic-bonded ferroelectrics, which may be extended to other van der Waals bilayer for design of non-volatile memory in future 2D wafers. Every intercalated adatom can be used to store one bit of data: "0" when binding to the down layer and "1" upon when binding to the up layer, giving rise to a possible approach of realizing single atom memory for high-density data storage.
Pub.: 27 Jul '17, Pinned: 30 Aug '17
Abstract: The ability of group III phosphates to adopt a two-dimensional van der Waals (2D VDW) structure observed for SiO2 was evaluated using density functional theory. The energies to form 2D hexagonal bilayers of corner-sharing tetrahedra did not follow a monotonic trend: the energies for AlPO4 and GaPO4 were similar to silica, while for BPO4 it was more than a factor of 2 larger and for InPO4 nearly another factor of 2 larger. The larger In atom favors octahedral coordination, accounting for the high energy of the 2D InPO4 structure. Meanwhile, boron’s small size leads to a different favored bulk structure than AlPO4 or GaPO4 which competes much more successfully with the 2D phase. The implication is a sweet spot in the cation size for forming 2D tetrahedral oxides that spans Si to Ga. The 2D BPO4 and GaPO4 structures displayed alternating rotations of the B(Ga)O4 and PO4 tetrahedra which allowed the B(Ga)–O–P bond angles to match those seen in the favored bulk compounds; no such rotations were required for 2D AlPO4 and SiO2 to match bond angles in bulk compounds. The interactions of AlPO4 and GaPO4 with Rh(111) as a prototypical growth substrate were also investigated with the results revealing adhesion dominated by VDW interactions. Alternate structures were considered with results mimicking those seen for SiO2: introducing larger rings of corner-sharing tetrahedra decreases the density, allowing the structure to be tuned by applying tensile strain. In comparison to SiO2, however, only even-membered rings are possible for the phosphates, restricting the range of structures and defects that can form. Finally, it was found that Mg2+ could readily replace Al3+ in AlPO4 in the process, creating ion exchange sites. The results highlight the great promise for adding AlPO4 and GaPO4 to the family of 2D VDW materials.
Pub.: 07 Jul '17, Pinned: 30 Aug '17
Abstract: Vertical stacking is widely viewed as a promising approach for designing advanced functionalities using two-dimensional (2D) materials. Combining crystallographically commensurate materials in these 2D stacks has been shown to result in rich new electronic structure, magnetotransport, and optical properties. In this context, vertical stacks of crystallographically incommensurate 2D materials with well-defined crystallographic order are a counterintuitive concept and, hence, fundamentally intriguing. We show that crystallographically dissimilar and incommensurate atomically thin MoS2 and Bi2Se3 layers can form rotationally aligned stacks with long-range crystallographic order. Our first-principles theoretical modeling predicts heterocrystal electronic band structures, which are quite distinct from those of the parent crystals, characterized with an indirect bandgap. Experiments reveal striking optical changes when Bi2Se3 is stacked layer by layer on monolayer MoS2, including 100% photoluminescence (PL) suppression, tunable transmittance edge (1.1→0.75 eV), suppressed Raman, and wide-band evolution of spectral transmittance. Disrupting the interface using a focused laser results in a marked the reversal of PL, Raman, and transmittance, demonstrating for the first time that in situ manipulation of interfaces can enable "reconfigurable" 2D materials. We demonstrate submicrometer resolution, "laser-drawing" and "bit-writing," and novel laser-induced broadband light emission in these heterocrystal sheets.
Pub.: 26 Jul '17, Pinned: 30 Aug '17
Abstract: MXenes are an interesting class of 2D materials consisting of transition metal carbides and nitrides, which are currently a subject of extensive studies. Although there have been theoretical calculations estimating the thermoelectric properties of MXenes, no experimental measurements have been reported so far. In this report, three compositions of Mo-based MXenes (Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx) have been synthesized and processed into free-standing binder-free papers by vacuum-assisted filtration, and their electrical and thermoelectric properties are measured. Upon heating to 800 K, these MXene papers exhibit high conductivity and n-type Seebeck coefficient. The thermoelectric power reaches 3.09 × 10–4 W m–1 K–2 at 803 K for the Mo2TiC2Tx MXene. While the thermoelectric properties of MXenes do not reach that of the best materials, they exceed their parent ternary and quaternary layered carbides. Mo2TiC2Tx shows the highest electrical conductivity in combination with the largest Seebeck coefficient of the three 2D materials studied.
Pub.: 05 Jul '17, Pinned: 30 Aug '17
Abstract: An increasing number of papers propose routes to implement thermal counterparts of electronic rectification. These schemes are mainly based on combinations of crystal anharmonicity and broken mirror symmetry. With respect to graphene, proposals pivot around shape asymmetry induced by using hetero-structures of nano-patterned or defected sections of pristine graphene. Using Molecular Dynamics (MD) we show that it suffices to split a graphene nano-ribbon into two unequal strained sections using external force which leads to large asymmetry in the forward and reverse heat fluxes. We find that the corresponding rectification ratio is enhanced by up to 60 %. Also, and more importantly, the polarity is controllable on-the-fly, i.e. by changing the position where force is applied. Based upon our results we propose a thermo-electric device which obviates the complex nano-patterning and lithography required to pattern graphene every time a new rectification value or sign is sought for, opening a route to simpler fabrication and characterization of phononic phenomena in 2D materials.
Pub.: 20 Jul '17, Pinned: 30 Aug '17
Abstract: ReS2 represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single-crystalline ReS2 in a distorted 1T phase is determined at room temperature for the in-plane directions parallel and perpendicular to the Re-chains, and the through-plane direction using time-domain thermoreflectance. ReS2 is prepared in the form of flakes having thicknesses of 60-450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in-plane thermal conductivity is higher along the Re-chains, (70 ± 18) W m(-1) K(-1) , as compared to transverse to the chains, (50 ± 13) W m(-1) K(-1) . As expected from the weak interlayer bonding, the through-plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m(-1) K(-1) , resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in-plane directions. The thermal conductivity and interface thermal conductance of ReS2 are discussed relative to the other 2D materials.
Pub.: 20 Jul '17, Pinned: 30 Aug '17
Abstract: As a new member of the MXene group, 2D Mo2 C has attracted considerable interest due to its potential application as electrodes for energy storage and catalysis. The large-area synthesis of Mo2 C film is needed for such applications. Here, the one-step direct synthesis of 2D Mo2 C-on-graphene film by molten copper-catalyzed chemical vapor deposition (CVD) is reported. High-quality and uniform Mo2 C film in the centimeter range can be grown on graphene using a Mo-Cu alloy catalyst. Within the vertical heterostructure, graphene acts as a diffusion barrier to the phase-segregated Mo and allows nanometer-thin Mo2 C to be grown. Graphene-templated growth of Mo2 C produces well-faceted, large-sized single crystals with low defect density, as confirmed by scanning transmission electron microscopy (STEM) measurements. Due to its more efficient graphene-mediated charge-transfer kinetics, the as-grown Mo2 C-on-graphene heterostructure shows a much lower onset voltage for hydrogen evolution reactions as compared to Mo2 C-only electrodes.
Pub.: 20 Jul '17, Pinned: 30 Aug '17
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