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Saiful I. Khondaker

Publications and source records attributed to Saiful I. Khondaker.

At least 19 recordsLinked to original sources
arXiv2025

Giant coercivity and enhanced intrinsic anomalous Hall effect at vanishing magnetization in a compensated kagome ferrimagnet

Ferrimagnets that can be driven to magnetic compensation show promise for use in spintronics as they exhibit a finite anomalous Hall effect at zero magnetic field without having a significant magnetic moment. Compensated ferrimagnet spintronic devices with both a large anomalous Hall effect and a high coercivity would be simultaneously easy to read and difficult to erase. The kagome ferrimagnet TbMn$_6$Sn$_6$ has been reported to host a large intrinsic anomalous Hall effect. Here, we demonstrate that doping the Mn sites with Cr drives the system towards magnetic compensation. For nearly compensated compositions at low temperatures, giant coercive fields exceeding 14 T are observed. Additionally, Cr doping significantly enhances the intrinsic anomalous Hall effect, which can be attributed to a shift in the Fermi level. Our results extend the range of unique magnetic states observed in kagome materials, demonstrating that chemical doping is an effective strategy to tune and realize these states.

cond-mat.str-el↗
arXiv2023

Observation of momentum-dependent charge density wave gap in a layered antiferromagnet GdTe3

Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The RTe3 (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material GdTe3, which is antiferromagnetic below 12 K, along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our Raman spectroscopy measurements show the presence of CDW amplitude mode at room temperature, which remains prominent when the sample is thinned down to 4-layers by exfoliation. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along G-Z and gradually decreases going towards G - M. Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions.

cond-mat.mes-hall↗
arXiv2023

Raman Study of Layered Breathing Kagome Lattice Semiconductor Nb3Cl8

Niobium chloride (Nb3Cl8) is a layered 2D semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb3Cl8 in bulk and few layer flakes using polarized Raman spectroscopy and density functional theory (DFT) analysis to determine the material's vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, 5 of which are A1g modes while the remaining 7 are Eg modes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and FWHM are affected. Raman measurements as a function of excitation wavelength (473-785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperature P3m1 phase to the low-temperature R3m phase. Magneto-Raman measurements carried out at 140 and 2 K between -2 to 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb3Cl8 material which can be further exploited for future applications.

cond-mat.mtrl-sci↗
arXiv2023

Observation of flat and weakly dispersing bands in a van der Waals semiconductor Nb3Br8 with breathing kagome lattice

Niobium halides, Nb3X8 (X = Cl,Br,I), which are predicted two-dimensional magnets, have recently gotten attention due to their breathing kagome geometry. Here, we have studied the electronic structure of Nb3Br8 by using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well explained by the theoretical calculations, which show they have Nb d character indicating their origination from the Nb atoms forming the breathing kagome plane. This van der Waals material can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as depicted from the time-dependent Raman spectroscopy measurements at room temperature. These results demonstrate that Nb3Br8 is an excellent material not only for studying breathing kagome induced flat band physics and its connection with magnetism, but also for heterostructure fabrication for application purposes.

cond-mat.mes-hall↗
arXiv2019

Uniform vapor pressure based CVD growth of MoS2 using MoO3 thin film as a precursor for co-evaporation

Chemical vapor deposition (CVD) is a powerful method employed for high quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for co-evaporation. Uniform evaporation rate of the MoO3 thin films provides uniform Mo vapor which promotes highly reproducible single crystal growth of MoS2 throughout the substrate. These high-quality crystals are as large as 95 um and were characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and transmission electron microscopy. The bottom gated field effect transistors fabricated using the as grown single crystals show n-type transistor behavior with a good on/off ratio of 10^6 under ambient condition. Our results presented here addresses the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS2 for future semiconductor device applications.

cond-mat.mtrl-sci↗
arXiv2014

Photoluminescence Quenching in Single-layer MoS2 via Oxygen Plasma Treatment

By creating defects via oxygen plasma treatment, we demonstrate optical properties variation of single-layer MoS2. We found that, with increasing plasma exposure time, the photoluminescence (PL) evolves from very high intensity to complete quenching, accompanied by gradual reduction and broadening of MoS2 Raman modes, indicative of distortion of the MoS2 lattice after oxygen bombardment. X-ray photoelectron spectroscopy study shows the appearance of Mo6+ peak, suggesting the creation of MoO3 disordered regions in the MoS2 flake. Finally, using band structure calculations, we demonstrate that the creation of MoO3 disordered domains upon exposure to oxygen plasma leads to a direct to indirect bandgap transition in single-layer MoS2, which explains the observed PL quenching.

cond-mat.mes-hall↗
arXiv2014

Photoluminescence quenching in gold - MoS2 hybrid nanoflakes

Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing. Here we consider pristine MoS2 and study its interaction with Au nanoislands, resulting in local variations of photoluminescence (PL) associated with various Au-MoS2 hybrid configurations. By controllably depositing monolayers of Au on MoS2 to form Au nanostructures of given size and thickness, we investigate the electronic structure of the resulting hybrid systems. We present strong evidence of PL quenching of MoS2 as a result of charge transfer from MoS2 to Au: p-doping of MoS2. The results suggest new avenues for 2D nanoelectronics, active control of transport or catalytic properties.

cond-mat.mes-hall↗
arXiv2014

Recent progress in parallel fabrication of individual single walled carbon nanotube devices

Single walled carbon nanotubes (SWNTs) have attracted immense research interest because of their remarkable physical and electronic properties. In particular, electronic devices fabricated using individual SWNT have shown outstanding device performance surpassing those of Si. However, for the widespread application of SWNTs based electronic devices, parallel fabrication techniques along with Complementary Metal Oxide (CMOS) compatibility are required. One technique that has the potential to integrate SWNTs at the selected position of the circuit in a parallel fashion is AC dielectrophoresis (DEP). In this paper, we review recent progress in the parallel fabrication of SWNT-based devices using DEP. The review begins with a theoretical background for the DEP and then discusses various parameters affecting DEP assembly of SWNTs. We also review the electronic transport properties of the DEP assembled devices and show that high performance devices can be fabricated using DEP. The technique for fabricating all semiconducting field effect transistor using DEP is also reviewed. Finally, we discuss the challenges and opportunities for the DEP assembly of SWNTs.

cond-mat.mes-hall↗
arXiv2014

Electrical property tuning via defect engineering of single layer MoS2 by oxygen plasma

We demonstrate that the electrical property of a single layer molybdenum disulfide (MoS2) can be significantly tuned from semiconducting to insulating regime via controlled exposure to oxygen plasma. The mobility, on-current and resistance of single layer MoS2 devices were varied up to four orders of magnitude by controlling the plasma exposure time. Raman spectroscopy, X-ray photoelectron spectroscopy and density functional theory studies suggest that the significant variation of electronic properties is caused by the creation of insulating MoO3-rich disordered domains in the MoS2 sheet upon oxygen plasma exposure, leading to an exponential variation of resistance and mobility as a function of plasma exposure time. The resistance variation calculated using an effective medium model is in excellent agreement with the measurements. The simple approach described here can be used for the fabrication of tunable two dimensional nanodevices on MoS2 and other transition metal dichalcogenides.

cond-mat.mes-hall↗
arXiv2013

High Performance Semiconducting Enriched Carbon Nanotube Thin Film Transistors Using Metallic Carbon Nanotube Electrode

High-performance solution-processed short-channel carbon nanotube (CNT) thin film transistors (TFTs) are fabricated using densely aligned arrays of metallic CNTs (m-CNTs) as source and drain electrodes, and aligned arrays of semiconducting enriched CNTs (s-CNTs) as channel material. The electrical transport measurements at room temperature show that the m-CNT contacted s-CNT array devices with a 2 um channel length perform superiorly to those of control Pd contacted s-CNT devices. The m-CNT contacted devices exhibit a maximum (average) on-conductance of 36.5 uS (19.2 uS), transconductance of 2.6 uS (1.2 uS), mobility of 51 cm2/Vs (25 cm2/Vs), and current on-off ratio of 1.1x10E6 (2.5x10E5). These values are almost an order of magnitude higher than that of control Pd contacted devices with the same channel length and s-CNT linear density. The low temperature charge transport measurements suggest that these improved performances are due to the lower charge injection barrier of m-CNT/s-CNT array devices compared to Pd/s-CNT array devices. We attribute the lower injection barrier to unique geometry of our devices. In addition to using semiconducting enriched CNT, our results suggest that using metallic CNT as an electrode can significantly enhance the performance of CNT TFTs.

cond-mat.mes-hall↗
arXiv2012

The Effect of Carbon Nanotube/Organic Semiconductor Interfacial Area on the Performance of Organic Transistors

We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0-30/μm) to the Pd electrodes. The average mobility is increased three, six and nine times for low, medium and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performance using reduced barrier height of SWCNT/pentacene interface.

cond-mat.mes-hall↗
arXiv2012

Efros-Shklovskii variable range hopping in reduced graphene oxide sheets of varying carbon sp2 fraction

We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.

cond-mat.mes-hall↗
arXiv2011

A general approach for high yield fabrication of CMOS compatible all semiconducting carbon nanotube field effect transistors

We report strategies of achieving both high assembly yield of carbon nanotubes at selected position of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using semiconducting enriched single walled carbon nanotube (s-SWNT) aqueous solution. When the DEP parameters were optimized for the assembly of individual s-SWNT, 97% of the devices show FET behavior with a maximum mobility of 210 cm2/Vs, on-off current ratio ~ 106 and on conductance up to 3 μS, however with an assembly yield of only 33%. As the DEP parameters were optimized so that 1-5 s-SWNTs are connected per electrode pair, the assembly yield was almost 90% with ~ 90% of these assembled devices demonstrating FET behavior. Further optimization gives an assembly yield of 100% with up to 10 SWNT/site, however with a reduced FET yield of 59%. Improved FET performance including higher current on-off ratio and high switching speed were obtained by integrating a local Al2O3 gate to the device. Our 90% FET with 90% assembly yield is the highest reported so far for carbon nanotube devices. Our study provides a pathway which could become a general approach for the high yield fabrication of CMOS compatible carbon nanotube FETs.

cond-mat.mes-hall↗
arXiv2011

Huge Volume Expansion and Structural Transformation of Carbon Nanotube Aligned Arrays during Electrical Breakdown in Vacuum

We observed a huge volume expansion of aligned single walled carbon nanotube (SWNT) arrays accompanied by structural transformation during electrical breakdown in vacuum. The SWNT arrays were assembled between prefabricated Pd source and drain electrodes of 2 μm separation on Si/SiO_2 substrate via dielectrophoresis. At high electrical field, the SWNT arrays erupt into large mushroom-like structure. Systematic studies with controlled electrical bias show that above a certain field SWNTs swell and transform to nanoparticles and flower-like structures with small volume increase. Further increase in electrical bias and repeated sweeping results into amorphous carbon as determined from scanning and transmission electron microscopy (TEM). Cross sectional studies using focused ion beam and TEM show the height of 2-3 nm SWNT array increased to about 1 μm with a volume gain of ~ 400 times. The electron energy loss spectroscopy reveals that graphitic sp^2 networks of SWNTs are transformed predominantly to sp^3. The current-voltage measurements also show an increase in the resistance of the transformed structure.

cond-mat.mes-hall↗
arXiv2011

Semiconducting enriched carbon nanotube align arrays of tunable density and their electrical transport properties

We demonstrate assembly of solution processed semiconducting enriched (99%) single walled carbon nanotubes (s-SWNT) in an array with varying linear density via ac-dielectrophoresis and investigate detailed electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density of the s-SWNTs in the array from 1/μm to 25 /μm. The maximum linear density of 25 s-SWNT /\mum reported here is the highest for any aligned semiconducting array. The DEP assembled s-SWNT devices provide opportunity to investigate transport property of the arrays in the direct transport regime. Room temperature electron transport measurements of the fabricated devices show that with increasing nanotube density the device mobility increases while the current on-off ratio decreases dramatically. For the dense array, the device current density was 16 μA/μm, on-conductance was 390 μS, and sheet resistance was 30 kΩ/\square. These values are the best reported so far for any semiconducting nanotube array.

cond-mat.mes-hall↗
arXiv2011

Coulomb Blockade and Hopping Conduction in Graphene Quantum Dots Array

We show that the low temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two dimensional array of graphene quantum dots (GQD). Below 15 K, a total suppression of current due to Coulomb blockade through GQD array was observed. Temperature dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2-10 meV corresponding to GQD sizes of 5-8 nm while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural and size induced disorder.

cond-mat.mes-hall↗
arXiv2011

Correlated breakdown of carbon nanotubes in an ultra-high density aligned array

We demonstrate that in a densely packed aligned array of single walled carbon nanotubes, the breakdown of one nanotube leads to a highly correlated breakdown of neighboring nanotubes, thereby producing a nano-fissure. We show that the origin of the correlation is the electrostatic field of the broken nanotubes that produces locally inhomogeneous current and Joule heating distributions in the neighboring intact nanotubes triggering their breakdowns in the vicinity of the broken nanotubes. Our results suggest that the densely aligned array behaves like a correlated solid.

cond-mat.mes-hall↗