Muhammad Zahir Iqbal

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.

Spin valve effect of NiFe/graphene/NiFe junctions

AbstractWhen spins are injected through graphene layers from a transition metal ferromagnet, high spin polarization can be achieved. When detected by another ferromagnet, the spin-polarized current makes high- and low-resistance states in a ferromagnet/graphene/ferromagnet junction. Here, we report manifest spin valve effects from room temperature to 10 K in junctions comprising NiFe electrodes and an interlayer made of double-layer or single-layer graphene grown by chemical vapor deposition. We have found that the spin valve effect is stronger with double-layer graphene than with single-layer graphene. The ratio of relative magnetoresistance increases from 0.09% at room temperature to 0.14% at 10 K for single-layer graphene and from 0.27% at room temperature to 0.48% at 10 K for double-layer graphene. The spin valve effect is perceived to retain the spin-polarized transport in the vertical direction and the hysteretic nature of magnetoresistance provides the basic functionality of a memory device. We have also found that the junction resistance decreases monotonically as temperature is lowered and the current-voltage characteristics show linear behaviour. These results revealed that a graphene interlayer works not as a tunnel barrier but rather as a conducting thin film between two NiFe electrodes.

Molecular n-doping of chemical vapor deposition grown graphene

It is essential to tailor the electronic properties of graphene in order to apply graphene films for use in electrodes. Here we report the modification of the electronic properties of single layer chemical vapor deposition (CVD) grown graphene by molecular doping without degrading its transparency and electrical properties. Raman spectroscopy and transport measurements revealed that p-toluenesulfonic acid (PTSA) imposes n-doping on single layer CVD grown graphene. The shift of G and 2D peak wave numbers and the intensity ratio of D and G peaks are analyzed as a function of reaction time. In the gate voltage dependent resistivity measurement, it is found that the maximum resistivity corresponding to the Dirac point is shifted toward a more negative gate voltage with increasing reaction time, indicating an n-type doping effect. We have also made single layer graphene p–n junctions by chemical doping and investigated the current–voltage characteristics at the p–n junction.

Effect of e-beam irradiation on graphene layer grown by chemical vapor deposition

We have grown graphene by chemical vapor deposition (CVD) and transferred it onto Si/SiO2 substrates to make tens of micron scale devices for Raman spectroscopy study. The effect of electron beam (e-beam) irradiation of various doses (600 to 12 000 μC/cm2) on CVD grown graphene has been examined by using Raman spectroscopy. It is found that the radiation exposures result in the appearance of the strong disorder D band attributed the damage to the lattice. The evolution of peak frequencies, intensities, and widths of the main Raman bands of CVD graphene is analyzed as a function of defect created by e-beam irradiation. Especially, the D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests transformation of graphene to the nanocrystalline and then to amorphous form. We have also estimated the strain induced by e-beam irradiation in CVD graphene. These results obtained for CVD graphene are in line with previous findings reported for the mechanically exfolia...

Improving the electrical properties of graphene layers by chemical doping

Abstract Although the electronic properties of graphene layers can be modulated by various doping techniques, most of doping methods cost degradation of structural uniqueness or electrical mobility. It is matter of huge concern to develop a technique to improve the electrical properties of graphene while sustaining its superior properties. Here, we report the modification of electrical properties of single- bi- and trilayer graphene by chemical reaction with potassium nitrate (KNO3) solution. Raman spectroscopy and electrical transport measurements showed the n-doping effect of graphene by KNO3. The effect was most dominant in single layer graphene, and the mobility of single layer graphene was improved by the factor of more than 3. The chemical doping by using KNO3 provides a facile approach to improve the electrical properties of graphene layers sustaining their unique characteristics.

Formation of p–n junction with stable p-doping in graphene field effect transistors using deep UV irradiation

We demonstrate the modification of the electronic properties of single layer chemical vapor deposition (CVD)-grown graphene by deep ultraviolet (DUV) light irradiation. The shift in the G and 2D bands in Raman spectra towards higher wavenumber suggests p-doping in graphene field effect transistors (FETs). In the transport measurements, the Dirac point is shifted towards positive gate voltage with increasing DUV light exposure time, revealing the strong p-doping effect without a large resistance increase. The doping is found to be stable in graphene devices, with a slight change in mobilities. We also constructed a p–n junction by DUV light exposure on selected regions of graphene, and investigated it with gate voltage dependent resistivity measurements and current–voltage characteristics.

Tuning the electrical properties of exfoliated graphene layers using deep ultraviolet irradiation

It is possible to tune the electrical properties of graphene layers in graphene-based nanoelectronic and optoelectronic devices while maintaining their unique band structure and electrical properties. We report here the use of deep ultraviolet irradiation (DUV) to tune the electronic properties of mechanically exfoliated single-layer graphene (SLG), bilayer graphene (BLG), and trilayer graphene (TLG). Raman spectroscopy and electrical transport measurements showed that DUV imposes p-doping on SLG, BLG, and TLG devices. The shift in the G and 2D peak wave numbers and intensity ratios of SLG, BLG, and TLG devices were examined as a function of irradiation time. Analysis of the shift in the Dirac points as a function of irradiation time indicated the p-type doping effect for all SLG, BLG, and TLG devices. This investigation has shown that DUV irradiation is a non-destructive approach which can be used to tailor the electrical properties of SLG, BLG, and TLG while maintaining their important structural and electrical characteristics.

Photocurrent Response of MoS2 Field-Effect Transistor by Deep Ultraviolet Light in Atmospheric and N2 Gas Environments

Molybdenum disulfide (MoS2), which is one of the representative transition metal dichalcogenides, can be made as an atomically thin layer while preserving its semiconducting characteristics. We fabricated single-, bi-, and multilayer MoS2 field-effect transistor (FET) by the mechanical exfoliation method and studied the effect of deep ultraviolet (DUV) light illumination. The thickness of the MoS2 layers was determined using an optical microscope and further confirmed by Raman spectroscopy and atomic force microscopy. The MoS2 FETs with different number of layers were assessed for DUV-sensitive performances in various environments. The photocurrent response to DUV light becomes larger with increasing numbers of MoS2 layers and is significantly enhanced in N2 gas environment compared with that in atmospheric environment.

Interlayer dependent polarity of magnetoresistance in graphene spin valves

We fabricated spin valves of NiFe/Al2O3/single layer graphene (SLG)/Co, NiFe/SLG/Co, and NiFe/Al2O3/Co. In the fabrication, lithography processes using e-beam resist or photoresist were avoided to obtain residue-free clean junctions in all devices. While all three types of magnetic junctions showed a distinctly clear spin valve effect from 10 to 300 K, the polarity of magnetoresistance (MR) revealed different signs. A negative MR value (−1.6% at 10 K) was observed in the spin valve effect for the junction with the Al2O3/SLG interlayer, however the other types of junctions showed conventional positive MR values. The positive or negative MR signs are explained by the Julliere model of spin-dependent tunneling.

Superior characteristics of graphene field effect transistor enclosed by chemical-vapor-deposition-grown hexagonal boron nitride

We report the characterization of high-quality chemical-vapor-deposition (CVD)-grown graphene devices on CVD-grown hexagonal boron nitride (h-BN). Electrical transport measurements and Raman spectroscopy showed that the graphene devices on the h-BN film presented superior carrier mobility and suppression of charged impurities. The hole mobility of graphene on h-BN and h-BN/graphene/h-BN at 4.2 K was 18000 and 20000 cm2 V−1 s−1, respectively. The CVD-grown h-BN not only provided an ideal substrate for graphene but also provided a protection layer against unwanted doping by O2. The h-BN/graphene/h-BN sandwich structure offers a significant advantage for the manufacturing of stable graphene electronic devices.