Papers by Sefaattin Tongay
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
Scientific Reports, Sep 13, 2013
Point defects in semiconductors can trap free charge carriers and localize excitons. The interact... more Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.
In the monolayer limit, transition metal dichalcogenides become direct-bandgap, light-emitting se... more In the monolayer limit, transition metal dichalcogenides become direct-bandgap, light-emitting semiconductors. The quantum yield of light emission is low and extremely sensitive to the substrate used, while the underlying physics remains elusive. In this work, we report over 100 times modulation of light emission efficiency of these two-dimensional semiconductors by physical adsorption of O 2 and/or H 2 O molecules, while inert gases do not cause such effect. The O 2 and/or H 2 O pressure acts quantitatively as an instantaneously reversible "molecular gating" force, providing orders of magnitude broader control of carrier density and light emission than conventional electric field gating. Physi-sorbed O 2 and/or H 2 O molecules electronically deplete ntype materials such as MoS 2 and MoSe 2 , which weakens electrostatic screening that would otherwise destabilize excitons, leading to the drastic enhancement in photoluminescence. In p-type materials such as WSe 2 , the molecular physisorption results in the opposite effect. Unique and universal in twodimensional semiconductors, the effect offers a new mechanism for modulating electronic interactions and implementing optical devices.
The stability and band bowing effects of two-dimensional transition metal dichalcogenide alloys M... more The stability and band bowing effects of two-dimensional transition metal dichalcogenide alloys MX 2(1Àx) X 0 2x (M ¼ Mo, W, and X, X 0 ¼ S, Se, Te) are investigated by employing the cluster expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0 K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8 eV to 1.0 eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics. V C 2013 American Institute of Physics. [http://dx.
Typical Raman spectra of transition-metal dichalcogenides (TMDs) display two prominent peaks, E 2... more Typical Raman spectra of transition-metal dichalcogenides (TMDs) display two prominent peaks, E 2g and A 1g , that are well separated from each other. We find that these modes are degenerate in bulk WSe 2 yielding one single Raman peak in contrast to other TMDs. As the dimensionality is lowered, the observed peak splits in two. In contrast, our ab initio calculations predict that the degeneracy is retained even for WSe 2 monolayers. Interestingly, for minuscule biaxial strain, the degeneracy is preserved, but once the crystal symmetry is broken by a small uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for uniaxially strained WSe 2 shows a good match to the measured Raman spectrum, which suggests that uniaxial strain exists in WSe 2 flakes, possibly induced during the sample preparation and/or as a result of the interaction between WSe 2 and the substrate. Furthermore, we find that WSe 2 undergoes an indirect-to-direct band-gap transition from bulk to monolayers, which is ubiquitous for semiconducting TMDs. These results not only allow us to understand the vibrational and electronic properties of WSe 2 , but also point to effects of the interaction between the monolayer TMDs and the substrate on the vibrational and electronic properties.
ZnMg and NbCl5 were intercalated in graphite and the presence of such molecules between the graph... more ZnMg and NbCl5 were intercalated in graphite and the presence of such molecules between the graphene sheets results in n- and p-type doping, respectively. The doping effect is confirmed by Hall and Raman measurements and the intercalation process is monitored by scanning tunneling microscopy. After intercalation the carrier concentration increase almost an order of magnitude and reaches values as high as 1019and 1018 cm−3 for p- and n-type doping, respectively. For higher intercalation times, the intercalated graphite turns back to be as ordered as pristine one as evidenced by the reduction in the D peak in Raman measurements. Intercalation compounds show remarkable stability allowing us to permanently tune the physical properties of few-layer graphite. Our study has provided a new route to produce stable and functional graphite intercalation compounds and the results can be applied to other graphitic structures such as few-layer graphene on SiC.
Nano Lett. 2012, 12, 5576−5580
The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides... more The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX 2 (M ¼ Mo, W; X ¼ S, Se, Te) are investigated from first principles calculations. The band alignments between different MX 2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering. V C 2013 American Institute of Physics. [http://dx.
Rectification and thermal stability of diodes formed at graphene/GaN interfaces have been investi... more Rectification and thermal stability of diodes formed at graphene/GaN interfaces have been investigated using Raman Spectroscopy and temperature-dependent current-voltage measurements. The Schottky barriers formed between GaN and mechanically transferred graphene display rectification that is preserved up to 550 K with the diodes eventually becoming non-rectifying above 650 K. Upon cooling, the diodes show excellent recovery with improved rectification. We attribute these effects to the thermal stability of graphene, which acts like an impenetrable barrier to the diffusion of contaminants across the interface, and to changes in the interface band alignment associated with thermally induced dedoping of graphene.
We report on the use of bromine intercalation of graphite to perform in situ tuning of the Schott... more We report on the use of bromine intercalation of graphite to perform in situ tuning of the Schottky barrier height (SBH) formed at many-layer-graphene (MLG) semiconductor interfaces. The intercalation of Br into MLG simultaneously increases interlayer separation between the graphene planes, while at the same time giving rise to an increase (decrease) in the free hole carrier density (Fermi energy) because of the transfer of electrons from carbon to bromine. The associated increase in the graphite work function results in an increase of the SBH, as manifested by lower forward/reverse current densities and higher depletion capacitances. These results are quantitatively understood within the context of the Schottky-Mott model and thermionic emission theory. The presented results have important implications for sensing and high power applications as well as the integration of carbon into semiconductors and carbon/graphene electronics.
We investigate the magnetic-field-and temperature-dependent transport properties of CVD-grown gra... more We investigate the magnetic-field-and temperature-dependent transport properties of CVD-grown graphene transferred to a flexible substrate (Kapton) and subjected to externally applied strain. In zero magnetic field, a logarithmic temperature-dependent conductivity correction, resulting from strong electron-electron interaction, becomes weaker with the application of strains as large as 0.6% because of an increased rate of chiral-symmetry-breaking scattering. With the application of a perpendicular magnetic field, we also observe positive magnetoconductance at low temperature (T = 5 K) due to weak localization. This magnetoconductance is suppressed with increasing strain, concomitant with a rapid decrease of the intervalley scattering rate (τ −1 i ). Our results are in good agreement with theoretical expectations and are consistent with a strain-induced decoupling between graphene and its underlying Kapton substrate.
ACS Nano 12, 5576−5580, 2012
Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandga... more Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe 2 , which shows stark contrast from the well-explored MoS 2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe 2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe 2 , identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.
Nano Letters, 6, 9095–9102 (2012)
"An improved process for graphene transfer was used to demonstrate high performance
graphene ena... more "An improved process for graphene transfer was used to demonstrate high performance
graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces
disorder and eliminates the polymeric residue that typically plagues transferred films. The
method also allows for purposely creating pores in the graphene of a controlled areal density.
Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene
source electrode is attributed to modulation of the contact barrier height between the
graphene and organic semiconductor due to a gate field induced Fermi level shift in the low
density of electronic-states graphene electrode. Pores introduced in the graphene source
electrode are shown to boost the G-VFET performance, which scales with the areal pore density
taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with
areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 106
and 200 mA/cm2, respectively, at drain voltages below 5 V."
We investigate the electronic transport properties across the pentacene/graphene interface. Curre... more We investigate the electronic transport properties across the pentacene/graphene interface. Current transport across the pentacene/graphene interface is found to be strikingly different from transport across pentacene/HOPG and pentacene/Cu interfaces. At low voltages, diodes using graphene as a bottom electrode display Poole–Frenkel emission, while diodes with HOPG and Cu electrodes are dominated by thermionic emission. At high voltages conduction is dominated by Poole–Frenkel emission for all three junctions. We propose that current across these interfaces can be accurately modeled by a combination of thermionic and Poole–Frenkel emission. Results presented not only suggest that graphene provides low resistive contacts to pentacene where a flat-laying orientation of pentacene and transparent metal electrodes are desired but also provides further understanding of the physics at the organic semiconductor/graphene interface.
We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumina... more We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native (undoped) device performance by a factor of 4.5 and is the highest PCE reported for graphene-based solar cells to date. Current–voltage, capacitance–voltage, and external quantum efficiency measurements show the enhancement to be due to the doping-induced shift in the graphene chemical potential that increases the graphene carrier density (decreasing the cell series resistance) and increases the cell’s built-in potential (increasing the open circuit voltage) both of which improve the solar cell fill factor.
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Papers by Sefaattin Tongay
graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces
disorder and eliminates the polymeric residue that typically plagues transferred films. The
method also allows for purposely creating pores in the graphene of a controlled areal density.
Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene
source electrode is attributed to modulation of the contact barrier height between the
graphene and organic semiconductor due to a gate field induced Fermi level shift in the low
density of electronic-states graphene electrode. Pores introduced in the graphene source
electrode are shown to boost the G-VFET performance, which scales with the areal pore density
taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with
areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 106
and 200 mA/cm2, respectively, at drain voltages below 5 V."
graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces
disorder and eliminates the polymeric residue that typically plagues transferred films. The
method also allows for purposely creating pores in the graphene of a controlled areal density.
Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene
source electrode is attributed to modulation of the contact barrier height between the
graphene and organic semiconductor due to a gate field induced Fermi level shift in the low
density of electronic-states graphene electrode. Pores introduced in the graphene source
electrode are shown to boost the G-VFET performance, which scales with the areal pore density
taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with
areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 106
and 200 mA/cm2, respectively, at drain voltages below 5 V."