Michael L. Jespersen

Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition

Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350u2009°C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

Predicting Vertical Phase Segregation in Polymer-Fullerene Bulk Heterojunction Solar Cells by Free Energy Analysis

Blends of poly(3-hexylthiophene) (P3HT) and C61-butyric acid methyl ester (PCBM) are widely used as a model system for bulk heterojunction active layers developed for solution-processable, flexible solar cells. In this work, vertical concentration profiles within the P3HT:PCBM active layer are predicted based on a thermodynamic analysis of the constituent materials and typical solvents. Surface energies of the active layer components and a common transport interlayer blend, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), are first extracted using contact angle measurements coupled with the acid-base model. From this data, intra- and interspecies interaction free energies are calculated, which reveal that the thermodynamically favored arrangement consists of a uniformly blended bulk structure capped with a P3HT-rich air interface and a slightly PCBM-rich buried interface. Although the bulk composition is solely determined by P3HT:PCBM ratio, composition near the buried interface is dependent on both the blend ratio and interaction free energy difference between solvated P3HT and PCBM deposition onto PEDOT:PSS. In contrast, the P3HT-rich overlayer is independent of processing conditions, allowing kinetic formation of a PCBM-rich sublayer during film casting due to limitations in long-range species diffusion. These thermodynamic calculations are experimentally validated by angle-resolved X-ray photoelectron spectroscopy (XPS) and low energy XPS depth profiling, which show that the actual composition profiles of the cast and annealed films closely match the predicted behavior. These experimentally derived profiles provide clear evidence that typical bulk heterojunction active layers are predominantly characterized by thermodynamically stable composition profiles. Furthermore, the predictive capabilities of the comprehensive free energy approach are demonstrated, which will enable investigation of structurally integrated devices and novel active layer systems including low band gap polymers, ternary systems, and small molecule blends.

Mobility enhancement in graphene transistors on low temperature pulsed laser deposited boron nitride

Low temperature pulsed laser deposited (PLD) ultrathin boron nitride (BN) on SiO2 was investigated as a dielectric for graphene electronics, and a significant enhancement in electrical transport properties of graphene/PLD BN compared to graphene/SiO2 has been observed. Graphene synthesized by chemical vapor deposition and transferred on PLD deposited and annealed BN exhibited up to three times higher field effect mobility compared to graphene on the SiO2 substrate. Graphene field effect transistor devices fabricated on 5u2009nm BN/SiO2 (300u2009nm) yielded maximum hole and electron mobility of 4980 and 4200u2009cm2/V s, respectively. In addition, significant improvement in carrier homogeneity and reduction in extrinsic doping in graphene on BN has been observed. An average Dirac point of 3.5u2009V and residual carrier concentration of 7.65u2009×u20091011u2009cm−2 was observed for graphene transferred on 5u2009nm BN at ambient condition. The overall performance improvement on PLD BN can be attributed to dielectric screening of charged im...

Temporally and spatially resolved plasma spectroscopy in pulsed laser deposition of ultra-thin boron nitride films

Physical vapor deposition (PVD) has recently been investigated as a viable, alternative growth technique for two-dimensional materials with multiple benefits over other vapor deposition synthesis methods. The high kinetic energies and chemical reactivities of the condensing species formed from PVD processes can facilitate growth over large areas and at reduced substrate temperatures. In this study, chemistry, kinetic energies, time of flight data, and spatial distributions within a PVD plasma plume ablated from a boron nitride (BN) target by a KrF laser at different pressures of nitrogen gas were investigated. Time resolved spectroscopy and wavelength specific imaging were used to identify and track atomic neutral and ionized species including B+, B*, N+, N*, and molecular species including N2*, N2+, and BN. Formation and decay of these species formed both from ablation of the target and from interactions with the background gas were investigated and provided insights into fundamental growth mechanisms of...

Impact of reduced graphene oxide on MoS2 grown by sulfurization of sputtered MoO3 and Mo precursor films

Monolayer molybdenum disulfide (MoS2), a two dimensional semiconducting dichalcogenide material with a bandgap of 1.8–1.9 eV, has demonstrated promise for future use in field effect transistors and optoelectronics. Various approaches have been used for MoS2 processing, the most common being chemical vapor deposition. During chemical vapor deposition, precursors such as Mo, MoO3, and MoCl5 have been used to form a vapor reaction with sulfur, resulting in thin films of MoS2. Currently, MoO3 ribbons and powder, and MoCl5 powder have been used. However, the use of ribbons and powder makes it difficult to grow large area-continuous films. Sputtering of Mo is an approach that has demonstrated continuous MoS2 film growth. In this paper, the authors compare the structural properties of MoS2 grown by sulfurization of pulse vapor deposited MoO3 and Mo precursor films. In addition, they have studied the effects that reduced graphene oxide (rGO) has on MoS2 structure. Reports show that rGO increases MoS2 grain growth...

Fast photo-switchable surfaces for boiling heat transfer applications

Several milligrams of the ruthenium-centered organometallic complex, ruthenium bis-4,4′-di(thiomethyl)-2,2′-bipyridine, mono-2u2009-(2-pyridyl)-1,3-oxathiane ([Ru{(HS-CH2)2-bpy}2{pox}](PF6)2) were synthesized and used to produce a self assembled monolayer film on a gold substrate. X-ray photoelectron spectroscopy analysis of the film detected the presence of bound thiolate, which is an indication of a chemisorbed film. Water contact angle measurements were performed before and after 5u2009min of visible light irradiation using an ozone-free 1000u2009W Xe(Hg) arc source with a 425-680u2009nm long pass mirror. The contact angle changed from 52° pre-irradiation (hydrophilic state) to 95° post-irradiation (hydrophobic state).

Versatile technique for assessing thickness of 2D layered materials by XPS

X-ray photoelectron spectroscopy (XPS) has been utilized as a versatile method for thickness characterization of various two-dimensional (2D) films. Accurate thickness can be measured simultaneously while acquiring XPS data for chemical characterization of 2D films having thickness up to approximately 10 nm. For validating the developed technique, thicknesses of few-layer graphene (FLG), MoS2 and amorphous boron nitride (a-BN) layer, produced by microwave plasma chemical vapor deposition (MPCVD), plasma enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD) respectively, were accurately measured. The intensity ratio between photoemission peaks recorded for the films (C 1s, Mo 3d, B 1s) and the substrates (Cu 2p, Al 2p, Si 2p) is the primary input parameter for thickness calculation, in addition to the atomic densities of the substrate and the film, and the corresponding electron attenuation length (EAL). The XPS data was used with a proposed model for thickness calculations, which was verified by cross-sectional transmission electron microscope (TEM) measurement of thickness for all the films. The XPS method determines thickness values averaged over an analysis area which is orders of magnitude larger than the typical area in cross-sectional TEM imaging, hence provides an advanced approach for thickness measurement over large areas of 2D materials. The study confirms that the versatile XPS method allows rapid and reliable assessment of the 2D material thickness and this method can facilitate in tailoring growth conditions for producing very thin 2D materials effectively over a large area. Furthermore, the XPS measurement for a typical 2D material is non-destructive and does not require special sample preparation. Therefore, after XPS analysis, exactly the same sample can undergo further processing or utilization.

Improvement in electrical properties of CVD graphene on low temperature pulsed laser deposited boron nitride on SiO 2 /Si substrate

Graphene has been extensively researched over the past decade due to its outstanding electrical, optical and mechanical properties. Since charge carriers in graphene are confined within one atomic layer thickness, their transport properties are easily influenced by the surrounding medium. Recently, significant enhancement in the transport properties of graphene has been observed as it forms layered heterostructure with hexagonal Boron Nitride (hBN), which offers an inert surface, high surface optical phonon modes for heat dissipation and a nearly lattice matched structure [1]. In general, the methods for synthesizing hBN require either high growth or high annealing temperature (~1000 °C) [2-3]. Pulsed laser deposition (PLD) offers an attractive alternative to overcome the high temperature requirement by increasing the excitations of deposited atoms and extending resonance time of the energetic species presence at the condensation surface. This facilitates low temperature growth of amorphous BN (a-BN) [4] which can then be phase transformed to polycrystalline hBN by low temperature annealing [5]. Here, we are reporting the electrical characterization of chemical vapor deposition (CVD) graphene on 5 and 30 nm BN grown on SiO2/Si substrate initially by PLD at 200 °C, and annealing at 400 °C for transformation to polycrystalline hBN. As synthesized BN was found to improve the electrical properties of graphene by significantly enhancing mobility, reducing carrier inhomogeneity and lowering extrinsic doping compared to graphene transferred on SiO2/Si substrate.