A pinboard by
Wei Sun Leong

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.


Reliable Piezoelectricity in Bilayer WSe2 for Piezoelectric Nanogenerators.

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

Carbon Nanotubes–Nanoporous Anodic Alumina Composite Membranes: Influence of Template on Structural, Chemical, and Transport Properties

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

Non-planar vertical photodetectors based on free standing two-dimensional SnS2 nanosheets.

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

Carbon Coated Alumina Nanofiber Membranes for Selective Ion Transport

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

High-Performance Polymers Sandwiched with Chemical Vapor Deposited Hexagonal Boron Nitrides as Scalable High-Temperature Dielectric Materials.

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

Chemically-Functionalized Phosphorene: Two-dimensional Multiferroics with Vertical Polarization and Mobile Magnetism.

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

Group III Phosphates as Two-Dimensional van der Waals Materials

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

Tunable and laser-reconfigurable 2D heterocrystals obtained by epitaxial stacking of crystallographically incommensurate Bi2Se3 and MoS2 atomic layers.

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