Scanning Tunneling Microscopes

STM

We report the effects of silver nanoparticles (Ag-NP) on morphological features of reduced graphene oxide (rGO) which show presence of atomic scale ripples and lattice defects. These entities present in the composite (Ag-rGO) modulates its magnetic and electrical properties. Graphene oxide (GO) and Ag-rGO are primarily paramagnetic while rGO is diamagnetic. Ag-rGO shows enhanced magnetization and hysteresis owing to the presence of defects. Magnetization is maximum in Ag-rGO synthesized using 1 mM of AgNO₃. Further increase in AgNO₃ concentration quenches magnetization due to excess C-O-C group present in Ag-rGO. Current-voltage (I-V) curves show transition from non-ohmic GO to ohmic rGO. In Ag-rGO, resistance decreases by at least four hundred times as compared to that of rGO and becomes n-type material. With increased Ag-NP concentration, rGO becomes susceptible to defects and loses its conductive nature. By optimizing Ag-NP concentration, both electrical and magnetic properties can be enhanced in Ag-rGO.

Nanodiamonds (ND) are carbon particles with a size less than 100 nm, formed by sp³ hybridization of carbon atoms. They have attracted considerable research interest for their potential applications ranging from sensing and bioimaging to quantum computation due to its physical, chemical and surface characteristics. In this work, we report the synthesis of ND from graphene dispersions by liquid-phase laser ablation technique, using an Nd: YAG laser. ND samples exhibit a characteristic photo-absorption at 226 nm, and it shows an excitation dependent emission similar to the carbon dots. Employing Scanning Tunneling Spectroscopy, the bandgap of ND is calculated to be 5.2 eV, and the work-function is around 4.2 eV. Further, we fabricated Resistive switching devices with ND film as the active layer, fluorine-doped tin oxide (FTO) as the bottom and silver (Ag) as the top electrodes. These FTO/ND/Ag devices show excellent memory characteristics with an on-off ratio of 10⁴ and reproducible cycling characteristics, demonstrating its potential for high-density memory applications.

Brain-inspired computation that mimics the coordinated functioning of neural networks through multitudes of synaptic connections is deemed to be the future of computation to overcome the classical von Neumann bottleneck. The future artificial intelligence circuits require scalable electronic synapse (e-synapses) with very high bit densities and operational speeds. In this respect, nanostructures of two-dimensional materials serve the purpose and offer the scalability of the devices in lateral and vertical dimensions. In this work, we report the nonvolatile bipolar resistive switching and neuromorphic behavior of molybdenum disulfide (MoS₂) quantum dots (QD) synthesized using liquid-phase exfoliation method. The ReRAM devices exhibit good resistive switching with an On–Off ratio of 10⁴, with excellent endurance and data retention at a smaller read voltage as compared to the existing MoS₂ based memory devices. Besides, we have demonstrated the e-synapse based on MoS₂ QD. Similar to our biological synapse, Paired Pulse Facilitation / Depression of short-term memory has been observed in these MoS₂ QD based e-synapse devices. This work suggests that MoS₂ QD has potential applications in ultra-high-density storage as well as artificial intelligence circuitry in a cost-effective way.

Graphene has been derived from graphite using a high-throughput liquid phase exfoliation technique, using two anionic surfactants with same polar head and different hydrophobic tail. Raman spectra indicate that the resulting samples are few-layer graphene, with large D-band, possibly arising from the surfactant-decorated edges. Scanning tunnelling spectroscopy (STS) demonstrates the linear density of states, with zero band gap to nearly 0.44 and 0.53 eV respectively for sodium dodecyl benzene sulfonate (SDBS) and dioctyl sodium sulfosuccinate (AOT) exfoliated graphene and the Fermi level has been slightly shifted towards the valence band due to the anionic surfactants. Thin film transistors (TFT’s) fabricated using these exfoliated graphene as channel layers exhibit ambipolar behaviour, with Dirac points shifted due to inherent charges in the layers. It is observed that the performance of transistors such as transconductance, carrier mobility, and Dirac point strongly depend on the surfactants used.

Bottom gate, top contact Organic Field Effect Transistors (OFETs) were fabricated using copper phthalocyanine (CuPc) as an active layer. The electrical properties of OFETs fabricated with CuPc annealed at different annealing temperatures and different channel length to width (L/W) ratios were studied. The transfer characteristics of the devices appear to improve with annealing temperature of CuPc and increasing L/W ratios of the devices. Upon annealing, the field effect mobility increased from 0.03 ± 0.004 cm²/V to 1.3 ± 0.02 cm²/V. Similarly, the interface state density reduced from 5.14 ± 0.39 × 1011 cm⁻²eV⁻² for the device fabricated using as deposited CuPc, to 2.41 ± 0.05 × 1011 /cm⁻²eV⁻² for the device with CuPc annealed at 80 °C. The on/off current ratio increased from 10² for the as-deposited device, to 10⁵ for the device with CuPc annealed at 80 °C. The dependence of the subthreshold swing on the L/W ratio was also investigated.

Pure and aluminum (Al) doped ZnO (Al:ZnO) nanorods (NRs) were deposited on silicon substrates by the hydrothermal method. The Al composition was kept at 2 % and 5 % for the Al:ZnO NR samples. The surface morphology and structural properties of the pure and Al:ZnO NRs were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The XRD study revealed the hexagonal phase of the ZnO with (101), (002) and (100) peaks and it also revealed that the major orientation of ZnO NRs were along the (002) planes. The SEM micrographs showed perfectly grown ZnO NRs with hexagonal shaped tips. The electrical characterization of the pure and Al:ZnO NR thin film surface was done by scanning tunneling microscopy (STM). Local electron spectroscopy was conducted to measure the tunneling current with respect to the applied bias. The n-type behavior and bandgap of the pure and Al:ZnO NRs were confirmed from the dI/dV – V characteristics. These studies are of fundamental importance for the fabrication of pure and Al:ZnO NR based nanodevices.

The stimuli-responsive orientation of liquid crystals having ionic appendages seems to have a great impact on the modern age molecular electronics. Low cost environmentally benign materials is the need of current century ailing from various severe environmental backlashes. Here we are presenting a stimuli-responsive ionic liquid crystal derived from an industrial waste by-product as a resistive memory material for soft and flexible electronics. Thermal and potential responsive ordering of this imidazolium-based ionic liquid crystal would be benefited in the design of high performance non-volatile memory devices. Potential-responsive ordering of the liquid crystalline mesophase is responsible for the orientation of charge carriers to improve the charge conduction of the system.

Vibration characteristics of a piezo crystal oscillator surface are studied using time series measurements of tunneling current. Using this technique, the fluctuations in the tunneling current between a scanning tunneling microscopy tip and the surface of a piezo crystal oscillator are studied, which reveal sub-nanometer vibrations with a sensitivity of 10^-2 ⁠Å / √Hz. As the excitation frequency applied to the crystal is varied, the vibrations on the oscillator surface exhibit a resonant response. Furthermore, we detected unconventional sub-nanometer perpendicular vibration modes excited on the crystal surface. These vibrations are in a direction transverse to the surface of the crystal oscillator, whose conventional vibration mode is in a horizontal plane parallel to the surface. We also find near resonance higher harmonics of the perpendicular mode. Thus, the piezo crystal oscillator together with the time series tunneling current measurements offer a convenient simultaneous drive and detection system with a wide operating frequency range.

Chemically reduced graphene oxide (rGO) samples with various degrees of reduction were prepared using hydrazine hydrate as the reducing agent. Scanning tunnelling microscope imaging shows that rGO contains rows of randomly distributed patches of epoxy groups. The local density of states of the rGO samples were mapped with scanning tunnelling spectroscopy, which shows that the bandgap in rGO originates from the epoxide regions itself. The Fermi level of the epoxide regions is shifted towards the valence band, making rGO locally p-type and a range of bandgaps from 0–2.2 eV was observed in these regions. Thin film transistors were fabricated using rGO as the channel layer. The devices show excellent output characteristics with clear saturation and gate dependence. The transfer characteristics show that rGO behaves as a p-type semiconductor; the devices exhibit an on/off ratio of 10⁴, with a low-bias hole mobility of 3.9 cm² / Vs.

An ambient atmosphere single quantum dot (QDs) rectifying diode with tunable threshold voltage has been fabricated using cobalt (Co) doped CdS QDs with a device structure of ITO/ZnO/QDs. Current–voltage (I–V) characterization of this device has been tested using ambient atmosphere scanning tunnelling microscope (STM). The scanning tunnelling spectra (STS) shows a very high rectification behavior of this single dot based device with a ratio of 103. The threshold voltage of this device decreases with increase in doping concentration of QDs. Reduction of this turn-on voltage occurs due to the formation of additional energy band of Co impurity within the band gap of QDs that exist closer to the valance band (VB) of CdS. Existence of this additional energy band has also been observed in the UV-VIS absorption data of Co doped CdS, which introduces an additional absorption peak in the near infrared region. This impurity band is fully populated at room temperature and the width of this band increases with doping concentration, which is the key for the tunability of threshold voltage. This finding has been explained with one empirical model of relative band shifting of semiconductor–QDs–tip interfaces with positive and negative substrate bias.

Graphene dispersions were prepared using the liquid phase exfoliation method with three different surfactants. One surfactant was used from each of the surfactant types, anionic, cationic, and non-ionic; those used, were sodium dodecylbenzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP), respectively. Raman spectroscopy was used to investigate the number of layers and the nature of any defects present in the exfoliated graphene. The yield of graphene was found to be less with the non-ionic surfactant, PVP. The deconvolution of 2D peaks at ~2700 cm^−1 indicated that graphene prepared using these surfactants resulted in sheets consisting of few-layer graphene. The ratio of intensity of the D and G bands in the Raman spectra showed that edge defect density is high for samples prepared with SDBS compared to the other two, and is attributed to the smaller size of the graphene sheets, as shown in the electron micrographs. In the case of the dispersion in PVP, it is found that the sizes of the graphene sheets are highly sensitive to the concentration of the surfactant used. Here, we have made an attempt to investigate the local density of states in the graphene sheets by measuring the tunnelling current–voltage characteristics. Graphene layers have shown consistent p-type behaviour when exfoliated with SDBS and n-type behaviour when exfoliated with CTAB, with a larger band gap for graphene exfoliated using CTAB. Hence, in addition to the known advantages of liquid phase exfoliation, we found that by selecting suitable surfactants, to a certain extent it is possible to tune the band gap and determine the type of majority carriers.

Catalyst-free (00 l) oriented ZnO nanorods (NRs) -based biosensor for the H₂O₂ sensing has been reported. The (002) oriented ZnO NRs as confirmed by X-ray diffraction were successfully grown on indium tin oxide (ITO) coated glass substrate by radio frequency (RF) sputtering technique without using any catalyst. Horseradish peroxidase (HRP) enzyme was immobilized on ZnO NRs by physical adsorption technique to prepare the biosensor. In this HRP/ZnO NR/ITO bioelectrode, nafion solution was added to form a tight membrane on surface. The prepared bioelectrode has been used for biosensing measurements by electrochemical analyzer. The electrochemical studies reveal that the prepared HRP/ZnO NR/ITO biosensor is highly sensitive to the detection of H₂O₂ over a linear range of 0.250–10 μM. The ZnO NR-based biosensor showed lower value of detection limit (0.125 μM) and higher sensitivity (13.40 µA/µM cm²) towards H₂O₂. The observed value of higher sensitivity attributed to larger surface area of ZnO nanostructure for effective loading of HRP besides its high electron communication capability. In addition, the biosensor also shows lower value of enzyme’s kinetic parameter (Michaelis–Menten constant, K m) of 0.262 μM which indicates enhanced enzyme affinity of HRP to H₂O₂. The reported biosensor may be useful for various applications in biosensing, clinical, food, and beverage industry.

Biosensor for the detection of hydrogen peroxide (H₂O₂) has been prepared by immobilizing horseradish peroxidase (HRP) enzyme using physical adsorption technique on zinc oxide (ZnO) nanostructures. The (002) oriented ZnO nanostructures as confirmed by X-ray diffraction, were successfully grown on indium tin oxide (ITO) coated glass substrate by pulsed laser ablation (PLA) without using any catalyst. The Nafion solution was added onto HRP/ ZnO/ ITO bio-electrode to form a tight membrane on the surface before carrying out bio-sensing measurements by electrochemical analyzer. The electrochemical studies reveal that the prepared bio-electrode HRP/ZnO/ITO is highly sensitive to the detection of H₂O₂ over a wide range of concentration with a linear range from 2.5 μM to 100 μM with the limit of detection 0.2 μM and sensitivity of 0.034 µA/ µM cm². The higher sensitivity attributed to larger surface area of ZnO nanostructure for effective loading of HRP besides its high electron communication capability. A relatively low value of the enzyme’s kinetic parameter (Michaelis-Menten constant, Kₘ) of 0.166 μM indicates enhanced enzyme affinity of HRP to H₂O₂. The reported biosensor may be useful for various applications in bio-sensing, clinical, food and beverage industry.

The biggest challenge in the resistive random access memory (ReRAM) technology is that the basic operational parameters, such as the set and reset voltages, the current on-off ratios (hence the power), and their operational speeds, strongly depend on the active and electrode materials and their processing methods. Therefore, for its actual technological implementations, the unification of the operational parameters of the ReRAM devices appears to be a difficult task. In this letter, we show that by fabricating a resistive memory device in a thin film transistor configuration and thus applying an external gate bias, we can control the switching voltage very accurately. Taking partially reduced graphene oxide, the gate controllable switching is demonstrated, and the possible mechanisms are discussed.

Synthesis of Zinc oxide (ZnO) nanoparticles has been carried out using wet chemical route at different annealing temperatures. The proper phase formation of ZnO nanoparticles was confirmed by XRD and the average particle size of ZnO particles was calculated using Scherer formula. It indicates that average particle size of ZnO particles increases on increasing the annealing temperature from 450 °C to 900 °C. The scanning electron microscopy (SEM) was performed to study the shape, size and morphology of synthesized ZnO nanoparticles. Further, the Fourier transform infrared spectroscopic (FTIR) analysis was carried to study the bonding vibrations which leads to the broad absorption band related to Zn–O vibrational band. The Photoluminescence (PL) characterization shows the three peaks having wavelength from 420–500 nm (UV region) corresponding to excitation wavelength of 320 nm. I–V characteristics of ZnO nanoparticles were also performed under constant current mode at Bias Voltage of –200 mV and tunneling current of 200 pA to study the semiconducting nature and conductivity.

Heteroatom polymeric precursor with an aromatic substituent (polyurethane acrylate) was designed and found to generate nitrogen-doped graphene at 1000 °C. Upon thermal degradation, a nitrogenous fragment was liberated in situ. As a consequence, nitrogen-doped graphene was formed in a single step. Further, this methodology was extended for studying the effect of annealing nonheteroatom polymer with an aromatic substituent (styrene butadiene random copolymer) under similar conditions. The isolated product demonstrated undoped graphene-like structure. The mechanism of nitrogen-doped graphitization was studied by annealing different heteroatom polymer units with an aliphatic and aromatic framework in a chemical vapor deposition reactor. The effect of precursor heteroatom polymer structure and annealing temperature in the presence of copper catalyst on the efficacy of formation of nitrogen-doped graphene was examined. The nitrogen-doped graphene exhibited semiconducting behavior over semimetallic behavior of the undoped graphene. Moreover, the nitrogen-doped graphene displayed better electrochemical properties compared to the undoped variety. These advances in the synthesis of graphitic structures can be readily extended for the fabrication of many advanced materials with controlled structure and properties.

The generation of stress in expanded graphite (E-GPT) due to covalent attachment of bulky side groups connected via a hetero atom is reported. Specifically, E-GPT is modified at different levels of grafting using “click” chemistry to graft 1-ethynyl-4-fluoro benzene onto graphene sheets via a triazole ring. In the range of grafting densitites examined, Raman spectroscopy indicates that the stress generated in graphene is linearly dependent on the extent of grafting. The functionalized graphene platelets with 6% functionalization transform from semi-metal behavior of the pristine material to semi-conductor behavior and indicates the ability of functionalization to change optical and electronic properties of graphene platelets similar to the deposition of thin layers of top gate oxides onto graphene.

Polyoligomeric silsesquioxane (POSS) decorated graphene nanoplatelets were synthesized using n-butyl lithium (n-BuLi). The isolated topological defects present along the edges of graphite played a crucial role in the grafting process. n-BuLi specifically scavenged the protons from these defect sites. These reactive anionic centres generated in situ, react with the POSS. Various microscopic characterization techniques indicated functionalization of the edges of graphene nanoplatelets. Further, highly exfoliated graphene layers were observed after modification. The present functionalization technique avoids the use of harsh oxidation techniques of graphite, which are commonly used for secondary modification of graphene.

We fabricate and explore the resonance characteristics of self-supporting thin film based metallic nanocantilever systems. Nanocantilevers of Au and Ag are fabricated from self-supporting (polycrystalline) thin films (∼100 nm) grown via a surfactant mediated process. Focused ion beam assisted milling and manipulation techniques are used to fabricate the nanocantilevers. The resonance characteristics of the cantilevers are investigated by the piezoelectric base excitation method and the frequencies of their first resonance modes are determined by digitally processing and analysing scanning electron microscopy images captured during the study. The resonance characteristics of the nanocantilevers are observed to deviate from the Euler–Bernoulli description. We suggest a polynomial expression to describe the peculiar dimensional dependence of resonance frequency of a mechanically vibrating nanocantilever, where the zeroth order term in the polynomial expression represents an Euler–Bernoulli like form. We also demonstrate a fabrication and measurement technique for an elastically coupled Au nanocantilever system and analysis of its vibration characteristics by means of an analogous mass–spring system.

The present pragmatic deals with the surface morphology and the temperature induced modifications of gold surface. The gold surface consists of three dimensional (3D) large nanoclusters and the shape of these nanoclusters was identified as cap like structure with approximately circular periphery. The effect of temperature on the gold surface has been characterized by Scanning Tunnelling Microscopy technique. Annealing the gold surface at 473 K induce inter-diffusion of the 3D-nanoclusters, while the formation of nanoscale step and terrace morphology near the cluster boundary has been detected at 573 K. This study also reveals that the clusters size and roughness of gold surface varies differently in different range of annealing temperature.

The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed. The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10^−10 torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms. The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition are found to be stable and could be used as a template for the growth of metallic nanostructures.