Sharafat Hossain

High Performance Graphene Nano-ribbon Thermoelectric Devices by Incorporation and Dimensional Tuning of Nanopores

Thermoelectric properties of Graphene nano-ribbons (GNRs) with nanopores (NPs) are explored for a range of pore dimensions in order to achieve a high performance two-dimensional nano-scale thermoelectric device. We reduce thermal conductivity of GNRs by introducing pores in them in order to enhance their thermoelectric performance. The electrical properties (Seebeck coefficient and conductivity) of the device usually degrade with pore inclusion; however, we tune the pore to its optimal dimension in order to minimize this degradation, enhancing the overall thermoelectric performance (high ZT value) of our device. We observe that the side channel width plays an important role to achieve optimal performance while the effect of pore length is less pronounced. This result is consistent with the fact that electronic conduction in GNRs is dominated along its edges. Ballistic transport regime is assumed and a semi-empirical method using Huckel basis set is used to obtain the electrical properties, while the phononic system is characterized by Tersoff empirical potential model. The proposed device structure has potential applications as a nanoscale local cooler and as a thermoelectric power generator.

Enhanced thermoelectric performance of graphene nanoribbon-based devices

There have been numerous theoretical studies on exciting thermoelectric properties of graphene nano-ribbons (GNRs); however, most of these studies are mainly based on simulations. In this work, we measure and characterize the thermoelectric properties of GNRs and compare the results with theoretical predictions. Our experimental results verify that nano-structuring and patterning graphene into nano-ribbons significantly enhance its thermoelectric power, confirming previous predictions. Although patterning results in lower conductance (G), the overall power factor (S2G) increases for nanoribbons. We demonstrate that edge roughness plays an important role in achieving such an enhanced performance and support it through first principles simulations. We show that uncontrolled edge roughness, which is considered detrimental in GNR-based electronic devices, leads to enhanced thermoelectric performance of GNR-based thermoelectric devices. The result validates previously reported theoretical studies of GNRs and d...

Monolayer MoS2 self-switching diodes

This paper presents a new molybdenum disulphide (MoS2) nanodevice that acts as a two-terminal field-effect rectifier. The device is an atomically-thin two-dimensional self-switching diode (SSD) that can be realized within a single MoS2 monolayer with very minimal process steps. Quantum simulation results are presented confirming the devices operation as a diode and showing strong non-linear I-V characteristics. Interestingly, the device shows p-type behavior, in which conduction is dominated by holes as majority charge carriers and the flow of reverse current is enhanced, while the flow of forward current is suppressed, in contrast to monolayer graphene SSDs, which behave as n-type devices. The presence of a large bandgap in monolayer MoS2 results in strong control over the channel, showing complete channel pinch-off in forward conduction, which was confirmed with transmission pathways plots. The device exhibited large leakage tunnelling current through the insulating trenches, which may have been due to...

Non-equilibrium tunneling in zigzag graphene nanoribbon break-junction results in spin filtering

Spintronic devices promise new faster and lower energy-consumption electronic systems. Graphene, a versatile material and candidate for next generation electronics, is known to possess interesting spintronic properties. In this paper, by utilizing density functional theory and non-equilibrium green function formalism, we show that Fano resonance can be generated by introducing a break junction in a zigzag graphene nanoribbon (ZGNR). Using this effect, we propose a new spin filtering device that can be used for spin injection. Our theoretical results indicate that the proposed device could achieve high spin filtering efficiency (over 90%) at practical fabrication geometries. Furthermore, our results indicate that the ZGNR break junction lattice configuration can dramatically affect spin filtering efficiency and thus needs to be considered when fabricating real devices. Our device can be fabricated on top of spin transport channel and provides good integration between spin injection and spin transport.

Graphene based single molecule detection based on thermoelectric power

Graphenes extraordinary electrical, electrochemical and structural property holds great promise for biological sensing applications. In most graphene based sensing studies, the changes in conductance are widely employed as a method for detection. Here, we propose the graphene thermoelectric property, such as the thermo power profile, can also be used for sensing purposes. We conducted a first principle study of the graphene thermoelectric sensing devices, employed the density functional theory combined with non-equilibrium green function formalism (DFT-NEGF). We found that the thermoelectric power of a graphene nano ribbon (GNR) device is significantly affected by surface attachment of molecules, which in turn can be used for single molecule sensing. Our study presents a simple thermoelectric based sensing device that may provide an alternative way of doing bio sensing.

Enhanced thermoelectric properties of engineered graphene nano-ribbons with nano-pores

In this paper we study the thermoelectric (TE) properties of graphene nano-ribbons (GNRs) with incorporated nanopores (NPs), and present a nanopore-engineering approach for enhancing their TE properties. The nearest neighbor tight binding (TB) model and Non equilibrium Greens function (NEGF) method were employed to obtain the electron transmission spectra. For phonon calculations, Tersoff potential along with Landaur formalism were used. We found a direct relationship between pore width and phononic thermal conductivity. The dependence of other parameters like Seebeck coefficient and electrical conductance on pore width was not so straight forward, and showed a clear dependence on the number of atoms in the side channel (NS). By optimizing NS we achieved a significant improvement in the thermoelectric figure of merit of GNRs-NPs. This research can be a route towards enhancing the TE properties of GNRs, making them potential candidates for future thermoelectronics.

Negative differential resistance in planar graphene quantum dot resonant tunneling diodes

Negative differential resistance (NDR), an electronic property present in resonant tunneling diodes, enables high performance terahertz frequency oscillators and multi-state logic and memory devices. An important measure of NDR is the peak-to-valley current ratio (PVCR) and this has been extremely lacking in solid-state NDR devices. Here we show how a dimensional mismatch between the quantum dot and the electrodes of a planar graphene Double Barrier Resonant Tunneling Diode (DB-RTD) greatly enhances the PVCR of the device up to a ratio of 103. Our findings suggest a promising future for the application of planar graphene quantum dot devices in next generation electronics.

Gate tunable graphene break junction spin filter

Graphene has attracted great interest for application in spintronics due to its intrinsic low spin-orbital and hyperflne interaction. The graphene spin filter, which permits the transport of electrons with certain spin only, has been used to realize electron spin-based logic devices. The spin polarity of a spin filter has strong correlation with the parallel or anti-parallel magnetic alignment of its electrodes. In this study, we theoretically investigate the effect on spin transport, of applying a voltage to a gate of a break junction based spin filter using density functional theory (DFT) combined with non-equilibrium greens function (NEGF). Our results indicate that an applied voltage to an electric gate induces an interference effect on the spin scattering states, which can be used to control the spin filtering polarity. This result indicates that a gate tunable spin filter is possible, which may unleash exciting opportunities for future spintronic circuits and systems.

Graphene nano-ribbon with nano-breaks as efficient thermoelectric device

It has been well established that delta-like transport distribution of electron gives the best thermoelectric performance. On another front, it has been experimentally verified that graphene nano-ribbon with nano-break in the channel region exhibits tunnelling. Here, we utilize the tunnelling phenomena observed in graphene break junctions to achieve delta like transport distribution. Indeed our device exhibit record ZT ranging from 10 to 100. This high ZT can be attributed to complete blockage of phonon transport due to the break. The electrical conductance also goes very low, however, near the tunnelling energy it becomes significant, giving rise to an enhanced ZT value. In this report we investigate the effect edge orientation and the width of the ribbon on thermoelectric property. Moreover, we investigate the effect of temperature on tunnelling and how it affect thermoelectric performance. We find that there is an optimal temperature at which the device performs best. In the simulations, we assumed ballistic transport and used first principle approach to obtain the electrical properties. The phononic system was characterized by a Tersoff empirical potential model. The proposed device structure has potential applications as a two-dimensional nanoscale local cooler and as a thermoelectric power generator when connected in arrays.

Graphene Field Effect Nanopore Glycine Detector

We present a new class of Graphene Nanopores that are tunable by means of a lateral in-plane field effect. The field effect is self-induced and does not require an additional gate terminal, and results in strong control over the channels conductivity. This capability can be used in order to tune the conductivity of the channel, making it comparable to the change in conductance induced by the translocation of a specific biomolecule through the Nanopore, leading to enhanced detection with very high sensitivity and specificity. Here, we present the use of this device for the detection of Glycine, an important biomarker of malignancy in early childhood brain-tumors, whose detection at very low levels can lead to early detection of cancerous brain-tumors and allow for their early removal. Quantum mechanical simulation results show that a translocation of a single Glycine molecule can be detected with more than 25% change in conductance, with high current levels near the microamps range and with very high specificity when present in aqueous solution.