Suchergebnisse

We present a simple and very convincing approach to visualizing that subsequent layers of graphene grow between the existing monolayer graphene and the copper catalyst in chemical vapor deposition (CVD). Graphene samples were grown by CVD and then transferred onto glass substrates by the bubbling method in two ways, either direct-transfer (DT) to yield poly (methyl methacrylate) (PMMA)/graphene/glass or (2) inverted transfer (IT) to yield graphene/PMMA/glass. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were used to reveal surface features for both the DT and IT samples. The results from FE-SEM and AFM topographic analyses of the surfaces revealed the underlayer growth of subsequent layers. The subsequent layers in the IT samples are visualized as 3D structures, where the smaller graphene layers lie above the larger layers stacked in a concentric manner. The results support the formation of the so-called "inverted wedding cake" stacking in multilayer graphene growth. ; MF thanks the Government of Lesotho, the University of Pretoria and the NRF for financial support for his study. A.T.C.J acknowledges support from the LRSM, through the U.S. National Science Foundation MRSEC, Grant No. DMR-1120901. ; http://scitation.aip.org/content/aip/journal/adva ; hb2014 ; ai2014

Kinetic modeling has been used to study the decomposition chemistry of ammonia at a wide range of temperatures, pressures, concentrations, and carrier gases mimicking the conditions in chemical vapor deposition (CVD) of metal nitrides. The modeling shows that only a small fraction of the ammonia molecules will decompose at most conditions studied. This suggests that the fact that the high NH3 to metal ratios often employed in CVD is due to the very low amount of reactive decomposition products being formed rather than due to rapid decomposition of ammonia into stable dinitrogen and dihydrogen as suggested by purely thermodynamic equilibrium models. ; Funding Agencies|Swedish foundation for Strategic Research through the project "Time-resolved low temperature CVD for III-nitrides" [SSF-RMA 15-0018]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at the Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Swedish Research Council (VR)Swedish Research Council

Alkylboranes, such as trimethylboron (TMB) and triethylboron (TEB), are promising alternative precursors in low temperature chemical vapor deposition (CVD) of boron-containing thin films. In this study, CVD growth of B-C films using TMB and quantum-chemical calculations to elucidate a gas phase chemical mechanism were undertaken. Dense, amorphous, boron-rich (B/C 1.5-3) films were deposited at 1000 degrees C in both dihydrogen and argon ambients, while films with crystalline B4C and B25C inclusions were deposited at 1100 degrees C in dihydrogen. A script-based automatization scheme was implemented for the quantum-chemical computations to enable time efficient screening of thousands of possible gas phase CVD reactions. The quantum-chemical calculations suggest TMB is mainly decomposed by an unimolecular alpha-H elimination of methane, which is complemented by dihydrogen-assisted elimination of methane in dihydrogen. ; Funding Agencies|European Spallation Source ERIC; Knut and Alice Wallenberg Foundation; German Science Foundation [GRK 1782]; Swedish Foundation for Strategic Research (SSF) [IS14-0027]; COST Action [MP1402]; COST (European Cooperation in Science and Technology); Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Chemical vapor deposition (CVD) is one of the most important techniques for depositing thin films of the group 13 nitrides (13-Ns), AIN, GaN, InN, and their alloys, for electronic device applications. The standard CVD chemistry for 13-Ns uses ammonia as the nitrogen precursor; however, this gives an inefficient CVD chemistry, forcing N/13 ratios of 100/1 or more. Here, we investigate the hypothesis that replacing the N-H bonds in ammonia with weaker N-C bonds in methylamines will permit better CVD chemistry, allowing lower CVD temperatures and an improved N/13 ratio. Quantum chemical computations show that while the methylamines have a more reactive gas-phase chemistry, ammonia has a more reactive surface chemistry. CVD experiments using methylamines failed to deposit a continuous film, while instead micrometer-sized gallium droplets were deposited. This study shows that the nitrogen surface chemistry is most likely more important to be considered than the gas-phase chemistry when searching for better nitrogen precursors for 13-N CVD. ; Funding Agencies|Swedish foundation for Strategic Research [SSF-RMA 15-0018]; Vinnova VINNMER Marie Curie incoming mobility program [2015-03714]; COST (European Cooperation in Science and Technology) [MP1402]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]

Synthesis of porous SiO2 thin films in room temperature was carried out using plasma enhanced chemical vapor deposition (CVD) in an electron cyclotron resonance microwave reactor with a downstream configuration.The gas adsorption properties and the type of porosity of the SiO2 thin films were assessed by adsorption isotherms of toluene at room temperature.The method could also permit the tailoring synthesis of thin films when both composition and porosity can be simultaneously and independently controlled. The result shows that it is possible to control the microstructure of oxide thin films deposited by room temperature plasma enhanced chemical vapor depositon (PECVD) by scarificial polymeric organic layers. ; Ministerio de Ciencia y Tecnología MAT2001-2820 ; European Union ENV4-CT97-0633

Aluminum nitride (AlN) is a semiconductor with a wide range of applications from light-emitting diodes to high-frequency transistors. Electronic grade AlN is routinely deposited at 1000 degrees C by chemical vapor deposition (CVD) using trimethylaluminum (TMA) and NH3, while low-temperature CVD routes to high-quality AlN are scarce and suffer from high levels of carbon impurities in the film. We report on an atomic layer deposition-like CVD approach with time-resolved precursor supply where readsorption of methyl groups from the AlN surface is suppressed by the addition of an extra pulse, H-2, N-2, or Ar, between the TMA and NH3 pulses. The suppressed readsorption allowed deposition of AlN films with a carbon content of 1 at. % at 480 degrees C. Kinetic and quantum-chemical modeling suggests that the extra pulse between TMA and NH3 prevents readsorption of desorbing methyl groups terminating the AlN surface after the TMA pulse. ; Funding Agencies|Swedish Foundation for Strategic Research (SSF) through the project "time-resolved low temperature CVD for III-nitrides" [SSF-RMA 15-0018]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty grant SFO Mat LiU) [2009 00971]; Swedish Research Council (VR)Swedish Research Council

Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This "pre-filling" of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process, which is also relevant to other 2D materials and their heterostructures. ; S.C. and R.W. acknowledge funding from EPSRC (Doctoral training award). R.S.W. acknowledges a Research Fellowship from St. John's College, Cambridge and a EU Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme. B.C.B. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 656214 - 2DInterFOX. B.C.B and J.C.M. acknowledge support from the Austrian Science Fund (FWF): P25721-N20 and the Austrian Research Promotion Agency (FFG): 848152 - GraphenMoFET. A.C.-V. acknowledges the Conacyt Cambridge Scholarship and Roberto Rocca Fellowship. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342). We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities at the BM20/ROBL beamline. We acknowledge the Helmholtz-Zentrum-Berlin Electron storage ring BESSY II for provision of synchrotron radiation at the ISISS beamline. We thank the ESRF and BESSY staff for continued support of our experiments and valuable discussion. ; This is the final version of the ...

Thin films of boron nitride (BN), particularly the sp(2)-hybridized polytypes hexagonal BN (h-BN) and rhombohedral BN (r-BN), are interesting for several electronic applications, given the bandgaps in the UV. They are typically deposited close to thermal equilibrium by chemical vapor deposition (CVD) at temperatures and pressures in the regions 1400-1800K and 1000-10000Pa, respectively. In this letter, the authors use the van der Waals corrected density functional theory and thermodynamic stability calculations to determine the stability of r-BN and compare it to that of h-BN as well as to cubic BN and wurtzitic BN. The authors find that r-BN is the stable sp(2)-hybridized phase at CVD conditions, while h-BN is metastable. Thus, their calculations suggest that thin films of h-BN must be deposited far from thermal equilibrium. ; Funding Agencies|Swedish Foundation for Strategic Research (SSF) [IS14-0027]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; SSF through the Future Research Leaders 6 grant; Swedish Research Council (VR) [2014-6336]; Marie Sklodowska Curie Actions [INCA 600398]; Kungl. Ingenjorsvetenskapsakademiens Hans Werthen-Fond

Knowledge of the structural evolution of thin films, starting by the initial stages of growth, is important to control the quality and properties of the film. The authors present a study on the initial stages of growth and the temperature influence on the structural evolution of sp(2) hybridized boron nitride (BN) thin films during chemical vapor deposition (CVD) with triethyl boron and ammonia as precursors. Nucleation of hexagonal BN (h-BN) occurs at 1200 degrees C on alpha-Al2O3 with an AlN buffer layer (AlN/alpha-Al2O3). At 1500 degrees C, h-BN grows with a layer-by-layer growth mode on AlN/alpha-Al2O3 up to similar to 4 nm after which the film structure changes to rhombohedral BN (r-BN). Then, r-BN growth proceeds with a mixed layer-by-layer and island growth mode. h-BN does not grow on 6H-SiC substrates; instead, r-BN nucleates and grows directly with a mixed layer-by-layer and island growth mode. These differences may be caused by differences in substrate surface temperature due to different thermal conductivities of the substrate materials. These results add to the understanding of the growth process of sp(2)-BN employing CVD. (C) 2015 American Vacuum Society. ; Funding Agencies|Swedish Research Council (VR) [621-2013-5585]; Carl Tryggers Stiftelse [12:175]; CeNano Program at Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; Bolyai Janos research scholarship of Hungarian Academy of Sciences

A comprehensive systematic method for chemical vapor deposition modeling consisting of seven well-defined steps is presented. The method is general in the sense that it is not adapted to a certain type of chemistry or reactor configuration. The method is demonstrated using silicon carbide (SiC) as a model system, with accurate matching to measured data without tuning of the model. We investigate the cause of several experimental observations for which previous research reports only have had speculative explanations. In contrast to previous assumptions, we can show that SiCl2 does not contribute to SiC deposition. We can confirm the presence of larger molecules at both low and high C/Si ratios, which have been thought to cause so-called step-bunching. We can also show that high concentrations of Si lead to other Si molecules other than the ones contributing to growth, which also explains why the C/Si ratio needs to be lower at these conditions to maintain high material quality as well as the observed saturation in deposition rates. Due to its independence of a chemical system and reactor configuration, the method paves the way for a general predictive CVD modeling tool. ; Funding Agencies|Swedish Foundation for Strategic Research project "SiC-the Material for Energy-Saving Power Electronics"Swedish Foundation for Strategic Research [EM11-0034]; Knut & Alice Wallenberg Foundation (KAW)Knut & Alice Wallenberg Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Swedish Research Council (VR)Swedish Research Council

The long-term (>18 months) protection of Ni surfaces against oxidation under atmospheric conditions is demonstrated by coverage with single-layer graphene, formed by chemical vapor deposition. In situ, depth-resolved X-ray photoelectron spectroscopy of various graphene-coated transition metals reveals that a strong graphene-metal interaction is of key importance in achieving this long-term protection. This strong interaction prevents the rapid intercalation of oxidizing species at the graphene-metal interface and thus suppresses oxidation of the substrate surface. Furthermore, the ability of the substrate to locally form a passivating oxide close to defects or damaged regions in the graphene overlayer is critical in plugging these defects and preventing oxidation from proceeding through the bulk of the substrate. We thus provide a clear rationale for understanding the extent to which two-dimensional materials can protect different substrates and highlight the key implications for applications of these materials as barrier layers to prevent oxidation. ; RSW acknowledges a Research Fellowship from St. John's College, Cambridge and a Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme. LD and SC acknowledge EPSRC Doctoral Training Awards and AC-V acknowledges a Conacyt Cambridge Scholarship and the Roberto Rocca Fellowship. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342). This research was partially supported by the EUFP7 Work Programme under grant GRAFOL (project reference 285275), and EPSRC under grant GRAPHTED (project reference EP/K016636/1). ; This is the final version of the article. It was first available from ACS via http://dx.doi.org/10.1021/jacs.5b08729

Recently, graphene-based organic light emitting diodes (OLEDs) were successfully demonstrated using graphene as anodes. However, the graphene electrodes have not been utilized for polymer light emitting diodes (PLEDs) yet, although the simpler device structure and the solution-based fabrication process of PLEDs are expected to be more advantageous in terms of time and cost. Here we demonstrate high-performance polymer light emitting diodes (PLEDs) with simple two-layer structures using interface-engineered single-layer graphene films as anodes. The single-layer graphene synthesized by chemical vapor deposition methods was transferred onto a glass substrate utilizing an elastic stamp, and its work function was engineered by varying the duration and the power of ultraviolet ozone (UVO) treatment. Thus, we were able to optimize the contact between silver electrodes and the graphene anodes, leading to the considerable enhancement of light-emitting performance. ; This work was supported by the OLED center of Samsung Display and Seoul National University, the Industrial Strategic Technology Development Program (KI002104, Development of Fundamental Technologies for Flexible Combined-Function Organic Electronic Device), the Global Research Lab (GRL) Program (2011-0021972), the Global Frontier Research Program (2011-0031627), and the Basic Science Research Program (2012M3A7B4049807, 2011-0017587 and 2009-0083540) through the National Research Foundation of Korea (NRF) funded by the Korean government (MEST and MKE). ; OAIID:oai:osos.snu.ac.kr:snu2013-01/102/0000029430/3 ; SEQ:3 ; PERF_CD:SNU2013-01 ; EVAL_ITEM_CD:102 ; USER_ID:0000029430 ; ADJUST_YN:Y ; EMP_ID:A076109 ; DEPT_CD:430 ; CITE_RATE:3.836 ; FILENAME:organic electronics 14, 2324 (sep, 2013).pdf ; DEPT_NM:전기·정보공학부 ; EMAIL:chlee7@snu.ac.kr ; SCOPUS_YN:Y ; CONFIRM:Y

Thin films of boron nitride in its sp(2)-hybridized form (sp(2)-BN) have potential uses in UV devices and dielectrics. Here, we explore chemical vapor deposition (CVD) of sp(2)-BN on various cuts of sapphire: Al2O3 ( 11 2 over bar 0 ), Al2O3 ( 1 1 over bar 02 ), Al2O3 ( 10 1 over bar 0 ), and Al2O3 (0001) using two CVD processes with two different boron precursors triethylborane and trimethylborane. Fourier transform infrared spectroscopy shows that sp(2)-BN grows on all the sapphire substrates; using x-ray diffraction, 2 theta/omega diffractogram shows that only Al2O3 ( 11 2 over bar 0 ) and Al2O3 (0001) rendered crystalline films: and using phi(phi)-scans, growth of the rhombohedral polytype (r-BN) films on these substrates is confirmed. These films were found to be epitaxially grown on an AlN interlayer with comparatively higher crystalline quality for the films grown on the Al2O3 ( 11 2 over bar 0 ) substrate, which is determined using omega(omega)-scans. Our study suggests that Al2O3 ( 11 2 over bar 0 ) is the most favorable sapphire substrate to realize the envisioned applications of r-BN films. ; Funding Agencies|Swedish Research Council [2017-04164]; Swedish Foundation for Strategic Research (SSF) [IS14-0027]; Carl Tryggers Foundation for Scientific Research [CTS 14:189]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Research Council VR-RFI [2019-00191]; Swedish Foundation for Strategic Research [RIF14-0053]

In this study we report on the effective p-doping of chemical vapor deposition (CVD) graphene transferred from copper foil onto SiO2/Si and PET substrates. Tetrachloroauric acid (HAuCl4) is used to promote graphene doping with a direct correlation between its concentration and modification of graphene's electrical properties. The doping mechanism entails the charge transfer (CT) between AuIII ions and graphene, and the formation of Au nanoparticles (AuNPs) on the surface. X-ray photoelectron spectroscopy (XPS) was employed to confirm this charge transfer, whereas the presence of the AuNPs was verified based on Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) images. The influence of doping on the electrical properties of graphene was assessed by Kelvin Probe Microscopy (KPM) and standard Hall Effect measurements, proving the ability of the method to effectively tune the carrier concentration, achieving sheet resistances as low as 79 Ω/sq. The controlled tuning of the electrical properties together with the use of flexible substrates makes the presented results a very interesting approach enabling the development of a variety of industrial applications, including flexible electronics. ; The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship. This work was also supported by the EU-FET grant GRAPHENICS618086 and the Polish National Science CentreUMO-2013/09/N/ST5/02481. ; Peer reviewed