Tunneling and Electronic Properties of Shape Controlled Mono and Heterogeneous Semiconductor Nanocrystals

Single NC electrical measurements of semiconductor NCs provide the fundamental limits for miniaturizing the semiconductor device thereby offering the ultimate control over the real time advanced device applications. In this context, STM and STS provide several advantages over conventional measurement techniques by performing the imaging and I-V spectroscopy down to atomic resolution limit. The corresponding tunneling characteristics are directly proportional to the density of states of the sample under characterization. Thus, by using STM and STS one can essentially probe the density of states of a material/molecule at the atomic resolution limit. This technique also surpasses the limitations imposed by statistical averaged optical spectroscopy techniques. Investigating the electronic properties of semiconductor monocomponent and heterogeneous NCs using STM and STS provides an opportunity to lead the material to advanced device applications like photovoltaics, optoelectronics and ultra high density memories. Semiconductor NCs are tiny single crystalline particles, which have the size of the order of few nanometers, exhibit size and shape dependent optical and electronic properties. In particular, the spatial confinement of the charge carriers in a semiconductor NC occurs in different directions depending on the shape of the NC and the extension of the electron and hole wave functions. Because of these variable degrees of freedom for charge carriers, a wide variety of novel aspects can be realized from such shape controlled semiconductor NCs. For example, a crossover in the valence levels upon changing the NC shape from 0D QDs to 1D nanorods result in a change in the polarization properties of emitted light. Shape of NCs is also one of important factor for field emission applications. Controlling the semiconductor NC to a 2D configuration is also an important criterion in the present and future electronic industry as they relatively eases the fabrication of complex device structures needed for miniaturization of electronic devices. Moreover, the shape controlled semiconductor NCs has crucial impacts in a wide variety of applications such as photovoltaics, optoelectronics, LEDs, NC lasers and biological labels etc. In addition to the shape controlled NCs, coupling of two different semiconducting quantum materials of different sizes or shapes known as hetero-junctions is found to be an unique approach in recent years to improve the tuning of electronic properties beyond the quantum confinement phenomenon. The main advantage of nanoscale hetero-junctions owes its origin to the formation of interface with two different materials which can lead to realization of new properties which are entirely different from the individual constituent materials. These nanoscale hetero-junctions are currently in the spotlight of research for improved performance in wide variety of applications. In this thesis work, we preferred the single NC electrical spectroscopy techniques like STM/STS and four probe measurements to investigate the tunneling and transport properties of these unique mono and heterogeneous semiconductor NCs.Research was carried out under the supervision of Prof. Somobrata Acharya of CAM under SPS [School of Physical Sciences]Research was conducted under the CSIR & DST gran

On-surface Double layer polymerization enhancing GNR lengths on an Au(111) surface

By performing controlled step-by-step annealing experiments of bilayers of GNR monomer reactants with multiple UHV-STM analysis of intermediate stages, we show that the coupling reaction takes place mainly in the uppermost layer of the monomer bilayer despite being separated from the Au(111) surface by the lowermost compact monomer carpet. This demonstrates that the initial monomer bilayer configuration plays acrucial role in lengthening the final GNR length once the intermediate dehalogenated polymer is cyclodehydrogenated. In this respect our experimental results directly provide the counter rationalization to the generalization of the metallic substrate catalytic role in the surface assisted coupling chemical reactions

On-surface Double layer polymerization enhancing GNR lengths on an Au(111) surface

By performing controlled step-by-step annealing experiments of bilayers of GNR monomer reactants with multiple UHV-STM analysis of intermediate stages, we show that the coupling reaction takes place mainly in the uppermost layer of the monomer bilayer despite being separated from the Au(111) surface by the lowermost compact monomer carpet. This demonstrates that the initial monomer bilayer configuration plays acrucial role in lengthening the final GNR length once the intermediate dehalogenated polymer is cyclodehydrogenated. In this respect our experimental results directly provide the counter rationalization to the generalization of the metallic substrate catalytic role in the surface assisted coupling chemical reactions

Long and isolated graphene nanoribbons by on-surface polymerization on Au(111)

Abstract Low electronic gap graphene nanoribbons (GNRs) are used for the fabrication of nanomaterial-based devices and, when isolated, for mono-molecular electronics experiences, for which a well-controlled length is crucial. Here, an on-surface chemistry protocol is monitored for producing long and well-isolated GNR molecular wires on an Au(111) surface. The two-step Ullmann coupling reaction is sequenced in temperature from 100 °C to 350 °C by steps of 50 °C, returning at room temperature between each step and remaining in ultrahigh vacuum conditions. After the first annealing step at 100 °C, the monomers self-organize into 2-monolayered nano-islands. Next, the Ullmann coupling reaction takes place in both 1st and 2nd layers of those nano-islands. The nano-island lateral size and shape are controlling the final GNR lengths. Respecting the above on-surface chemistry protocol, an optimal initial monomer coverage of ~1.5 monolayer produces isolated GNRs with a final length distribution reaching up to 50 nm and a low surface coverage of ~0.4 monolayer suitable for single molecule experiments

On-surface Double layer polymerization enhancing GNR lengths on an Au(111) surface

By performing controlled step-by-step annealing experiments of bilayers of GNR monomer reactants with multiple UHV-STM analysis of intermediate stages, we show that the coupling reaction takes place mainly in the uppermost layer of the monomer bilayer despite being separated from the Au(111) surface by the lowermost compact monomer carpet. This demonstrates that the initial monomer bilayer configuration plays acrucial role in lengthening the final GNR length once the intermediate dehalogenated polymer is cyclodehydrogenated. In this respect our experimental results directly provide the counter rationalization to the generalization of the metallic substrate catalytic role in the surface assisted coupling chemical reactions

Tunneling electronic excitations spatial mapping of a single graphene nanoribbon on Ag(111)

Using low temperature scanning tunneling microscopy (STM), spectroscopy (STS), and precise dI/dVmapping in combination with density-functional theory-parametrized semiempirical calculations, we reportand discuss the origin of tunneling electronic excitations that occur along a seven-carbon-atom-wide armchairgraphene nanoribbon (7-aGNR) physisorbed on Ag(111) as compared to a reference on Au(111) surface. Onboth surfaces, on-surface synthesized 7-aGNRs of variable lengths are selectively chosen for performing theSTM and STS (dI/dV ) excitation mapping along a truly isolated molecule. For exactly the same 7-aGNRmolecule length, the difference in work functions between Ag(111) and Au(111) generates different edge stateselectronic configuration that result in a curved molecular conformation on Ag(111) and a strictly flat one onAu(111). At the interface between the 7-aGNR and Ag(111), this curved conformation produces an additionalset of quantum-box-like tunneling resonances that partially mix with the intrinsic 7-aGNR tunneling excitations

Short-Lived, Intense and Narrow Bluish-Green Emitting Gold Zinc Sulfide Semiconducting Nanocrystals

In nanoscale, gold is one of the widely studied metals. It is well-known for its size dependent surface plasmonic absorbance. It has also been reported that clusters of a few atoms of gold can show fluorescence. However, these optical properties of gold are mostly associated with Au(0), and little has been explored for the compounds of gold in nanoscale. Herein, we report a new semiconducting nanocrystalline material involving Au­(I), which shows intense, narrow, and stable emission at its bandedge absorption. These are composed of Au, Zn, and S and synthesized by introducing Au to zinc sulfide or Zn to gold­(I) sulfide nanocrystals in their aqueous dispersion and under ambient condition. The obtained emission is short-lived and tunable in a short spectral window. These new semiconducting fluorescent gold based nanomaterials are characterized with UV–visible, photoluminescence spectroscopy, TCSPC, HRTEM, STM, and STS experiments. Further, the electrical and optical sensing properties of these nanocrystals have also been measured

Current rectification by a single ZnS nanorod probed using a scanning tunneling microscopic technique

We report on the rectification properties from a single ZnS nanorod measured using the UHV-SPM technique. The rectification behavior is evidenced from the current-voltage characteristics measured on a single ZnS nanorod. We propose a tunneling mechanism where the direct tunneling mechanism is dominant at lower applied bias voltages followed by resonant tunneling through discrete energy levels of the nanorod. A further increase in the bias voltage changes the tunneling mechanism to the Fowler-Nordheim tunneling regime enabling rectification behavior. Realizing rectification from a single ZnS nanorod may provide a means of realizing a single nanorod based miniaturized device

Planar bridging an atomically precise surface trench with a single molecular wire on an Au(1 1 1) surface

International audienceIn a bridge configuration, a single graphene nanoribbon (GNR) is positioned with a picometer precision over a trench in between two monoatomic steps on an Au(111) surface. This GNR molecular wire adopts a deformed conformation towards the down terrace in between the two contact step edges. Using differential conductance dI/dV mapping from a low-temperature scanning tunneling microscope, it is demonstrated how the electronic delocalization along GNR is cut at each contact by its down curvature. It points out the need to bring conductive nanocontacts backside of the support for preserving the front side GNR planar conformation

A Bottom-Up Approach toward Fabrication of Ultrathin PbS Sheets

Two-dimensional (2D) sheets are currently in the spotlight of nanotechnology owing to high-performance device fabrication possibilities. Building a free-standing quantum sheet with controlled morphology is challenging when large planar geometry and ultranarrow thickness are simultaneously concerned. Coalescence of nanowires into large single-crystalline sheet is a promising approach leading to large, molecularly thick 2D sheets with controlled planar morphology. Here we report on a bottom-up approach to fabricate high-quality ultrathin 2D single crystalline sheets with well-defined rectangular morphology via collective coalescence of PbS nanowires. The ultrathin sheets are strictly rectangular with 1.8 nm thickness, 200-250 nm width, and 3-20 mu m length. The sheets show high electrical conductivity at room and cryogenic temperatures upon device fabrication. Density functional theory (DFT) calculations reveal that a single row of delocalized orbitals of a nanowire is gradually converted into several parallel conduction channels upon sheet formation, which enable superior in-plane carrier conduction