We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters-arm length and arm diameter, respectively-but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.

Excited-state interaction between CdSe quantum dots (QDs) of different sizes (2.3, 3.2, and 4.2 nm diameter) and graphene oxide (GO) was probed by depositing them as films on conducting glass electrodes. The emission of smaller CdSe QDs (2.3 nm) was quenched by GO three times faster than that of larger QDs (4.2 nm). Electrophoretic deposition allowed us to sequentially deposit single or multiple layers of different sized QDs and GO assemblies on conducting glass electrodes and to modulate the photoresponse in photoelectrochemical solar cells. Superior photoconversion efficiency through the incorporation of GO was attributed to improved charge separation in the composite assembly.

CdS, CdSe, and ternary CdSexS(1-x) are some of the most widely studied II-VI semiconductors due to their broad range of applications and promising performance in numerous systems. One-dimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire, which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the [001] direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ∼10 ps with increasing Se; however, the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics that give them broad potential applications in the fields of optoelectronics, and photovoltaics.

Our recent efforts have focused on utilizing graphene based assemblies for light energy conversi on, energy storage and sensing applications [1-4]. Electrophoretic deposition method has been employed to anchor single-layered GO onto to TiO 2 NP films. Under UV irradiation photogenerated electrons are transfe rred across graphene sheet. These electrons are capable of reducing Ag+ ions from solution and deposit Ag nanoparticles on the GO surface. By controlling the irradiation time we can tune the size and loading o f metal nanoparticles. The semiconductor-graphene-metal nanoparticle (SGM) films can be used as a photocatalyst or in se nsing applications. SGM films tested for SERRS enhancemen t of the porphyrin target molecule, TAPP showed over three orders of magnitude enhancement. Immersing th e SGM films in a solution containing TAPP results in interaction with RGO and this gets concentrated nea r the preexisting Ag nanoparticles. Increased concentrati on and SERS enhancement makes the detection of nanomolar concentration of molecules that exist in solution. In addition, these composites can also be used to develop new photocatalyst devices for water splitti ng reaction. The role of Graphene based assemblies in promoting charge separation will be discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Graphene oxide (GO) serves as a two-dimensional carbon nano-mat to anchor catalyst nanoparticles. We have developed a photocatalyst assembly by anchoring ZnO and Ag nanoparticles on graphene oxide sheets suspended in ethanol. Upon photoirradiation, the electrons are transferred from ZnO to GO to produce reduced graphene oxide (RGO). The ZnO–RGO composites are further decorated with Ag nanoparticles by reducing Ag+ ions quantitatively with excess electrons stored in RGO. Under continuous UV-illumination we observe charging of ZnO nanoparticles as evidenced by the shift in absorption edge. However, no shift in the band edge is seen for ZnO–RGO or ZnO–RGO–Ag composites under UV irradiation indicating the quick discharge of electrons on RGO surface. Such charge–discharge phenomenon on the graphene oxide sheet was further probed by carrying out reduction of methyl viologen. Improved charge separation and selectivity in the reduction process was achieved in these graphene based photocatalytic assemblies.

Photochemically generated methyl viologen radicals undergo electron transfer with graphene oxide (GO) in ethanol suspensions. This charge transfer interaction results in the reduction of GO as well as storage of electrons. The stored electrons can be utilized to reduce Ag+ ions and thus anchor silver nanoparticles on reduced graphene oxide (RGO). The spectroscopic experiments that elucidate the quantitative electron transfer and transmission electron microscopy that highlights the potential of designing metal–RGO assemblies are discussed.