Browsing by Subject "Chemical vapor deposition"

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Open Access
A bean-like formation of germanium nanoparticles inside CNTs by the subsequent operation of colloidal synthesis and catalytic chemical vapor deposition methods
(Wiley, 2018) Karatutlu, Ali; Boi, F. S.; Wilson, R. M.; Ersoy, O.; Ortac, Bülend; Sapelkin, A.
The first attempts of implanting Ge nanoparticles (Ge NPs) inside iron filled CNTs (IF-CNTs) by a subsequent use of the bench top colloidal synthesis and chemical vapor deposition (CVD) approach is shown. Ge NPs are colloidally synthesized (with a 3.8 ± 0.6 nm in size) before the deposition. The hybrid Ge NPs/IF-CNTs structure and morphology are characterized using high-resolution transmission electron microscopy, scanning electron microscopy, selective area electron diffraction, and X-ray diffraction studies. After the deposition, Ge NPs appear to be grown in size and to be sprinkled almost homogeneously into the IF-CNTs similar to a bean-like deposition. CNTs diameter is also identified to be enlarged drastically when using Ge NPs as a catalyst in CVD compared to the CNTs formation without Ge NPs. In addition, micro-length rectangular Ge µPs are also found outside the nanotube core. Rietveld analysis shows the presence of γ-Fe (Fm-3m), ferromagnetic α-Fe (Im-3m), Fe3C, Ge (Fd-3m), and multiwall CNTs. The results indicate that Ge NPs and IF-CNTs demonstrate cocatalytic activity in increasing the respective sizes, which are dramatically larger than those obtained by the conventional approaches.
Open Access
CVD grown 2D MoS2 layers: a photoluminescence and fluorescence lifetime imaging study
(Wiley-VCH Verlag, 2016) Özden, A.; Şar, H.; Yeltik A.; Madenoğlu, B.; Sevik, C.; Ay, F.; Perkgöz, N. K.
In this letter, we report on the fluorescence lifetime imaging and accompanying photoluminescence properties of a chemical vapour deposition (CVD) grown atomically thin material, MoS2. µ-Raman, µ-photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM) are utilized to probe the fluorescence lifetime and photoluminescence properties of individual flakes of MoS2 films. Usage of these three techniques allows identification of the grown layers, grain boundaries, structural defects and their relative effects on the PL and fluorescence lifetime spectra. Our investigation on individual monolayer flakes reveals a clear increase of the fluorescence lifetime from 0.3 ns to 0.45 ns at the edges with respect to interior region. On the other hand, investigation of the film layer reveals quenching of PL intensity and lifetime at the grain boundaries. These results could be important for applications where the activity of edges is important such as in photocatalytic water splitting. Finally, it has been demonstrated that PL mapping and FLIM are viable techniques for the investigation of the grain-boundaries. (Figure presented.). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Open Access
Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, Necmi
The influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Electrically controlled terahertz spatial light modulators with graphene arrays
(IEEE, 2016) Kakenov, Nurbek; Takan, T.; Özkan, V. A.; Balcı, Osman; Polat, Emre Ozan; Altan, H.; Kocabaş, Coşkun
Gate-tunable high-mobility electrons on atomically thin graphene layers provide a unique opportunity to control electromagnetic waves in a very broad spectrum. In this paper, we describe an electrically-controlled multipixel terahertz light modulators. The spatial light modulator is fabricated using two large-area graphene layers grown by chemical vapor deposition and transferred on THz transparent and flexible substrates. Room temperature ionic liquid, inserted between the graphene, provides mutual gating between the graphene layers. We used passive matrix addressing to control local charge density thus the THz transmittance. With this device configuration, we were able to obtain 5×5 arrays of graphene modulator with 65% modulation between 0.1 to 1.5 THz.
Open Access
Experimental and theoretical studies of transport through large scale, partially aligned arrays of single-walled carbon nanotubes in thin film type transistors
(2007) Kocabas, C.; Pimparkar, N.; Yesilyurt O.; Kang, S.J.; Alam, M.A.; Rogers J.A.
Gate-modulated transport through partially aligned films of single-walled carbon nanotubes (SWNTs) in thin film type transistor structures are studied experimentally and theoretically. Measurements are reported on SWNTs grown by chemical vapor deposition with systematically varying degrees of alignment and coverage in transistors with a range of channel lengths and orientations perpendicular and parallel to the direction of alignment. A first principles stick-percolation-based transport model provides a simple, yet quantitative framework to interpret the sometimes counterintuitive transport parameters measured in these devices. The results highlight, for example, the dramatic influence of small degrees of SWNT misalignment on transistor performance and imply that coverage and alignment are correlated phenomena and therefore should be simultaneously optimized. The transport characteristics reflect heterogeneity in the underlying anisotropic metal-semiconductor stick-percolating network and cannot be reproduced by classical transport models. © 2007 American Chemical Society.
Open Access
Femtosecond laser crystallization of amorphous Ge
(American Institute of Physics, 2011) Salihoglu, O.; Kürüm, U.; Yaglıoglu, G. H.; Elmali, A.; Aydınlı, Atilla
Ultrafast crystallization of amorphous germanium (a-Ge) in ambient has been studied. Plasma enhanced chemical vapor deposition grown a-Ge was irradiated with single femtosecond laser pulses of various durations with a range of fluences from below melting to above ablation threshold. Extensive use of Raman scattering has been employed to determine post solidification features aided by scanning electron microscopy and atomic force microscopy measurements. Linewidth of the Ge optic phonon at 300 cm -1 as a function of laser fluence provides a signature for the crystallization of a-Ge. Various crystallization regimes including nanostructures in the form of nanospheres have been identified.
Open Access
Generation of InN nanocrystals in organic solution through laser ablation of high pressure chemical vapor deposition-grown InN thin film
(Springer, 2012-07-27) Alkis, S.; Alevli, M.; Burzhuev, S.; Vural, H. A.; Okyay, Ali Kemal; Ortaç, B.
We report the synthesis of colloidal InN nanocrystals (InN-NCs) in organic solution through nanosecond pulsed laser ablation of high pressure chemical vapor deposition-grown InN thin film on GaN/sapphire template substrate. The size, the structural, the optical, and the chemical characteristics of InN-NCs demonstrate that the colloidal InN crystalline nanostructures in ethanol are synthesized with spherical shape within 5.9-25.3, 5.45-34.8, 3.24-36 nm particle-size distributions, increasing the pulse energy value. The colloidal InN-NCs solutions present strong absorption edge tailoring from NIR region to UV region.
Open Access
Graphene mode-locked Cr:LiSAF laser at 850 nm
(OSA - The Optical Society, 2015) Canbaz F.; Kakenov, N.; Kocabas, C.; Demirbas, U.; Sennaroglu, A.
We report, for the first time to our knowledge, a mode-locked femtosecond Cr:LiSAF laser initiated with a high-quality monolayer graphene saturable absorber (GSA), synthesized by chemical-vapor deposition. The tight-focusing resonator architecture made it possible to operate the Cr:LiSAF laser with only two 135 mW, 660 nm low-cost single-mode diode lasers. At a pump power of 270 mW, the laser produced nearly transform-limited 68 fs pulses with an average power of 11.5 mW at 850 nm. The repetition rate was around 132 MHz, corresponding to a pulse energy and peak power of 86 pJ and 1.26 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be sustained for hours. The saturation fluence and the modulation depth of the GSA were further determined to be 28 μJ/cm2 and 0.62%, respectively. 2015 Optical Society of America.
Open Access
Growth of vertically aligned carbon nanotubes over self-ordered nano-porous alumina films and their surface properties
(Elsevier, 2012) Rana, K.; Kucukayan-Dogu, G.; Bengu, E.
Nanoporous anodic aluminum oxide (AAO) with self-organized arrays of uniform nanopores have been used for various applications in the fields of sensing, storage, separation and template-based fabrication of metal nanowires, carbon nanotubes, oxides and polymers. The work presented here involves the production and use of AAO templates for growth of aligned multi walled carbon nanotube arrays. AAO templates were formed by electrochemical oxidation of aluminum in different electrolyte solutions containing sulfuric, oxalic and phosphoric acid. SEM was used for the analysis of the surface morphology of the AAO films. The porous structures with pore size in the range of 25–120 nm were observed. Pore sizes were correlated with the type of acidic solutions used as the electrolyte. Finally, AAO surfaces have been used as substrates for the growth of vertically aligned carbon nanotubes through chemical vapor deposition technique, which showed super-hydrophobic behavior as confirmed by contact angle measurements.
Open Access
High-Speed InSb photodetectors on GaAs for mid-IR applications
(IEEE, 2004) Kimukin, I.; Bıyıklı, Necmi; Kartaloǧlu, T.; Aytür, O.; Özbay, Ekmel
We report p-i-n type InSb-based high-speed photodetectors grown on GaAs substrate. Electrical and optical properties of photodetectors with active areas ranging from 7.06 × 10 -6 cm 2 to 2.25 × 10 -4 cm 2 measured at 77 K and room temperature. Detectors had high zero-bias differential resistances, and the differential resistance area product was 4.5 Ω cm 2. At 77 K, spectral measurements yielded high responsivity between 3 and 5 μm with the cutoff wavelength of 5.33 μm. The maximum responsivity tor 80-μm diameter detectors was 1.00 × 10 5 V/W at 435 μm while the detectivity was 3.41×10 9 cm Hz 1/2/W. High-speed measurements were done at room temperature. An optical parametric oscillator was used to generate picosecond full-width at half-maximum pulses at 2.5 μm with the pump at 780 mm. 30-μm diameter photodetectors yielded 3-dB bandwidth of 8.5 GHz at 2.5 V bias.
Open Access
Improving performance and stability in quantum dot-sensitized solar cell through single layer graphene/Cu2S nanocomposite counter electrode
(Elsevier, 2020) Akman, E.; Altıntaş, Y.; Gülen, M.; Yılmaz, M.; Mutlugün, Evren; Sönmezoğlu, S.
In this work, we presented an effective nanocomposite to modify the Cu2S film by employing single layer graphene (SLG) frameworks via chemical vapor deposition, and utilized this nanocomposite as counter electrode (CE) with CdSe/ZnS core/shell quantum dots for highly stable and efficient quantum dot-sensitized solar cell (QDSSC). Furthermore, Cu2S film is directly synthesized on SLG framework by electrodeposition method. Using this nanocomposite as CE, we have achieved the high efficiency as high as 3.93% with fill factor of 0.63, which is higher than those with bare Cu2S CE (3.40% and 0.57). This remarkable performance is attributed to the surface area enhancement by creating nanoflower-shape, the reduction of charge transfer resistance, improvement of catalytic stability, and the surface smoothness as well as good adhesion. More importantly, no visible color change and detachment from surface for the Cu2S@SLG nanocomposite was observed, demonstrating that the SLG framework is critical role in shielding the Cu2S structure from sulphur ions into electrolyte, and increasing the adhesion of the Cu2S structure on surface, thus preventing its degradation. Consequently, the Cu2S@SLG nanocomposite can be utilized as an effective agent to boost up the performance of QDSSCs.
Open Access
Kinetic analysis of 2D Mo2C crystal growth via CVD
(Institute of Physics Publishing Ltd., 2023-09-28) Buke, G. C.; Çaylan, Ömer Refet; Ogurtani, O. T.
We investigated the growth mechanism of 2D Mo2C crystals by chemical vapor deposition (CVD) under various time and temperature conditions. The growth kinetics and mechanism of Mo2C on Cu via chemical vapor deposition (CVD) were investigated using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. To analyze the surface coverage, we employed scanning electron microscopy (SEM) and applied the modified JMAK model to determine the growth rate and activation energy. The growth rate of Mo2C exhibited temperature-dependent behavior, described by the Arrhenius relationship, with an apparent activation energy of 4 eV. The Avrami plot exhibited an exponent of 3 indicating a complex process with nucleation and growth.
Open Access
Lateral overgrowth of germanium for monolithic integration of germanium-on-insulator on silicon
(Elsevier, 2015) Hyung Nam J.; Alkis, S.; Nam, D.; Afshinmanesh F.; Shim J.; Park, J.; Brongersma, M.; Okyay, Ali Kemal; Kamins, T.I.; Saraswat, K.
A technique to locally grow germanium-on-insulator (GOI) structure on silicon (Si) platform is studied. On (001) Si wafer, silicon dioxide (SiO2) is thermally grown and patterned to define growth window for germanium (Ge). Crystalline Ge is grown via selective hetero-epitaxy, using SiO2 as growth mask. Lateral overgrowth of Ge crystal covers SiO2 surface and neighboring Ge crystals coalesce with each other. Therefore, single crystalline Ge sitting on insulator for GOI applications is achieved. Chemical mechanical polishing (CMP) is performed to planarize the GOI surface. Transmission electron microscopy (TEM) analysis, Raman spectroscopy, and time-resolved photoluminescence (TRPL) show high quality crystalline Ge sitting on SiO2. Optical response from metal-semiconductor-metal (MSM) photodetector shows good optical absorption at 850 nm and 1550 nm wavelength. © 2015 Elsevier B.V. All rights reserved.
Open Access
Low dark current metal-semiconductor-metal photodiodes based on semi-insulating GaN
(AIP Publishing LLC, 2006) Bütün, S.; Gökkavas, M.; Yu, H.; Özbay, Ekmel
Metal-semiconductor-metal photodetectors on semi-insulating GaN templates were demonstrated and compared with photodetectors fabricated on regular GaN templates. Samples were grown on a metal organic chemical vapor deposition system. Devices on semi-insulating template exhibited a dark current density of 1.96 × 10-10 A/cm2 at 50 V bias, which is four orders of magnitude lower compared with devices on regular template. Device responsivities were 101.80 and 88.63 A/W at 50 V bias for 360 nm ultraviolet illumination for semi-insulating and regular templates, respectively. Incident power as low as 3 pW was detectable using the devices that were fabricated on the semi-insulating template. © 2006 American Institute of Physics.
Open Access
Microcavity enhanced amorphous silicon photoluminescence
(IEEE, 1997) Serpengüzel, Ali; Aydınlı, Atilla; Bek, Alpan
A microcavity enhancement of room temperature photoluminescence (PL) of a hydrogenated amorphous silicon (a-Si:h) was performed. A quantum confinement model was developed to describe the occurrence of the PL in the bulk a-Si:H. According to the model, small a-Si clusters are in a matrix of a-Si:H. The regions with Si-H, having larger energy gaps due to strong Si-H bonds, isolate these clusters, and form barrier regions around them. The PL originates from these a-Si clusters.
Open Access
Nanoscale tribology of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates
(Cambridge University Press, 2016) Demirbaş, T.; Baykara, M. Z.
We present a comprehensive nanoscale tribological characterization of single-layer graphene grown by chemical vapor deposition (CVD) and transferred onto silicon oxide (SiO2) substrates. Specifically, the nanotribological properties of graphene samples are studied via atomic force microscopy (AFM) under ambient conditions using calibrated probes, by measuring the evolution of friction force with increasing normal load. The effect of using different probes and post-transfer cleaning procedures on frictional behavior is evaluated. A new method of quantifying lubrication performance based on measured friction coefficient ratios of graphene and SiO2 is introduced. A comparison of lubrication properties with mechanically-exfoliated graphene is performed. Results indicate that CVD-grown graphene constitutes a very good solid lubricant on SiO2, reducing friction coefficients by ∼ 90% for all investigated samples. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values is quantitatively analyzed, with results revealing a substantial increase in friction on these structural defects.
Open Access
Nanotribological properties of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates
(2015) Demirbaş, Tuna
To extend the lifespan of mechanical systems, wear and friction must be minimized with the utilization of lubricants. On the other hand, traditional fluid-based lubrication schemes fail in nano- and micro-scale systems due to increasing surface-to-volume ratios and associated physical effects. As such, research efforts in recent years have been aimed at characterizing the structure and mechanical properties of various candidates for solid lubricants. Due to its outstanding electronic and mechanical properties, the two-dimensional “wonder material” graphene has been the focus of a large variety of experiments in the past decade. Based on its promise as a single-layer solid lubricant suitable for use in nano- and micro-scale systems, the nanotribological properties of graphene have been investigated in several studies in the literature. While frictional characteristics of mechanically exfoliated graphene samples as a function of layer number have been related to the effect of puckering, the nanotribological behavior of graphene samples grown by chemical vapor deposition (CVD) is still under investigation. Considering that high quality graphene of sufficient dimensions for practical applications is currently grown by CVD and requires transfer from metal foils onto various substrates, the need for an extensive understanding of the nanotribological properties of such graphene samples arises. Based on the discussion above, this M.S. thesis presents a comprehensive structural and nanotribological characterization of CVD-grown graphene transferred onto oxidized silicon substrates (SiO2/Si). In particular, the processes of sample preparation and post-preparation transfer onto SiO2/Si substrates are optimized via a series of experiments. Advanced microscopy techniques are utilized for the structural and morphological characterization of the obtained graphene films. In particular, optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) are used to inspect graphene coverage on the substrate and associated structural features. On the other hand, Raman spectroscopy is employed to confirm the single-layer character of CVD-grown samples. The nanotribological properties of CVD-grown graphene samples on SiO2/Si are studied by AFM in the friction force microscopy (FFM) mode under ambient conditions by measuring the evolution of friction force with increasing normal load. The effect of using different probe tips, growth conditions, and post-transfer cleaning procedures on frictional behavior is evaluated. A comparison of lubrication performance with mechanically-exfoliated graphene is also performed. Results indicate that CVD-grown graphene acts as a very good solid lubricant on SiO2/Si, reducing coefficients of friction by ~90% for all investigated samples. It is shown that as-transferred CVD-grown graphene exhibits the highest mean lubrication performance and that the associated values drop slightly with post-transfer cleaning. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values are quantitatively evaluated, with results revealing a substantial increase in friction on these structural defects.
Open Access
A near-infrared range photodetector based on indium nitride nanocrystals obtained through laser ablation
(IEEE, 2014) Tekcan, B.; Alkis, S.; Alevli, M.; Dietz, N.; Ortac, B.; Bıyıklı, Necmi; Okyay, Ali Kemal
We present a proof-of-concept photodetector that is sensitive in the near-infrared (NIR) range based on InN nanocrystals. Indium nitride nanocrystals (InN-NCs) are obtained through laser ablation of a high pressure chemical vapor deposition grown indium nitride thin film and are used as optically active absorption region. InN-NCs are sandwiched between thin insulating films to reduce the electrical leakage current. Under-1 V applied bias, the recorded photoresponsivity values within 600-1100-nm wavelength range are as high as (3.05 × 10-2) mA/W. An ultrathin layer of nanocrystalline InN thin film is, therefore, a promising candidate for NIR detection in large area schemes. © 2014 IEEE.
Open Access
Synthesis of graphene on ultra-smooth copper foils for large area flexible electronics
(IEEE, 2015) Polat, E. O.; Balcı, Osman; Kakenov, Nurbek; Kocabaş, Coşkun; Dahiya, R.
This work demonstrates the synthesis of high quality, single layer graphene on commercially available ultra-smooth copper foils. The presented method will result in improved scalability of graphene based electronic and optical devices. Our approach is compatible with roll-to-roll printing as well as transfer printing of graphene layers on to a broad range of substrates including flexible and ultra-thin polymers. We propose that using commercially available ultra-smooth coppers provides scalable approach with the reduced variation of transport properties sourced from local graphene quality.
Open Access
Visible photoluminescence from low temperature deposited hydrogenated amorphous silicon nitride
(Pergamon Press, 1996) Aydınlı, A.; Serpengüzel, A.; Vardar, D.
Hydrogenated amorphous silicon nitride (a-SiNx:H) samples have been prepared by plasma enhanced chemical vapor deposition (PECVD) using a mixture of silane (SiH4), nitrogen and ammonia (NH3). Most films exhibit visible photoluminescence (PL) and some emit strong PL after annealing. While films grown without NH3 exhibit PL in the deep red, those grown with NH3 show PL in the green. The PL properties of these films with no oxygen (O) content are similar to those of silicon oxide (SiOx) films and porous Si. Using infrared and X-ray Photoelectron Spectroscopy, we suggest that PL from a-SiNx:H films originate from Si clusters which form during PECVD and crystallize upon annealing. We propose that the presence of O is not necessary for efficient PL.