Ali Hossain Khan

Colloidal Synthesis of Strongly Fluorescent CsPbBr3 Nanowires with Width Tunable down to the Quantum Confinement Regime

We report the colloidal synthesis of strongly fluorescent CsPbBr3 perovskite nanowires (NWs) with rectangular section and with tuneable width, from 20 nm (exhibiting no quantum confinement, hence emitting in the green) down to around 3 nm (in the strong quan-tum-confinement regime, emitting in the blue), by introducing in the synthesis a short acid (octanoic acid or hexanoic acid) together with alkyl amines (octylamine and oleylamine). Temperatures below 70 {\deg}C promoted the formation of monodisperse, few unit cell thick NWs that were free from byproducts. The photoluminescence quantum yield of the NW samples went from 12% for non-confined NWs emitting at 524 nm to a maximum of 77% for the 5 nm diameter NWs emitting at 497 nm, down to 30% for the thinnest NWs (diameter ~ 3nm), in the latter sample most likely due to aggregation occurring in solution.

Demonstration of Ultrarapid Interfacial Formation of 1D Fullerene Nanorods with Photovoltaic Properties

We demonstrate ultrarapid interfacial formation of one-dimensional (1D) single-crystalline fullerene C60 nanorods at room temperature in 5 s. The nanorods of ∼ 11 μm in length and ∼ 215 nm in diameter are developed in a hexagonal close-pack crystal structure, contrary to the cubic crystal structure of pristine C60. Vibrational and electronic spectroscopy provide strong evidence that the nanorods are a van der Waals solid, as evidenced from the preservation of the electronic structure of the C60 molecules within the rods. Steady state optical spectroscopy reveals a dominance of charge transfer excitonic transitions in the nanorods. A significant enhancement of photogenerated charge carriers is observed in the nanorods in comparison to pristine C60, revealing the effect of shape on the photovoltaic properties. Due to their ultrarapid, large-scale, room-temperature synthesis with single-crystalline structure and excellent optoelectronic properties, the nanorods are expected to be promising for photosensitive devices applications.

Size controlled ultranarrow PbS nanorods: spectroscopy and robust stability

We describe a simple route for designing ultranarrow PbS rods of 1.7 nm in diameter in a single step under bench-top reaction conditions. The rods are size controlled into 2.5 nm diameter by tuning the reaction temperature. Both the nanorods exhibit molecule-like discrete narrow optical behavior with high fluorescence quantum yield. The confinement owing to the change in diameter is reflected in absorption and photoluminescence spectroscopy. The strength of the exciton–phonon coupling has been evaluated from the Frohlich coupling and Huang–Rhys parameter in the adiabatic approximation using Raman spectroscopy, which reveals a different degree of confinement in size controlled rods. The thermogravimetric analysis reveals robustness of these nanorods. Such ultranarrow size controlled nanorods are of fundamental scientific interest and in principle can find potential applications in nanorod based devices. The nanorods with strong discrete fluorescence and robust stability could be useful in biological labeling, fluorescence resonance energy transfer and optoelectronic applications.

Shape control of zincblende CdSe nanoplatelets

The lateral dimensions of CdSe nanoplatelets have a strong and unique influence on their opto-electronic properties, with sizes that can be tuned from the weak to the strong exciton confinement regime. There are state-of-the-art reports on several nanoplatelet syntheses; however, at present only the thickness is well-controlled. We demonstrate here that we can achieve a control over the aspect ratio and overall nanoplate area by carefully adjusting the reagents that induce the in-plane growth. A variation of the fraction of hydrated Cd(OAc)2 in a Cd(OAc)2/Cd(OAc)2·2H2O mixture tailors the nanoplatelet aspect ratio. This occurs independently of the reaction time, which can be used to fine-tune the overall length and width. An interpretation is given by the in situ formation of a small amount of hydroxide anions that alter the surface energy of specific planes.

Mechanism of Versatile Catalytic Activities of Quaternary CuZnFeS Nanocrystals Designed by a Rapid Synthesis Route

Quaternary alloyed nanocrystals (NCs) composed of earth abundant, environment friendly elements are of interest for energy-harvesting applications. These complex NCs are useful as catalysts for the degradation of multiple refractory organic pollutants as well as nitro-organic reduction at a rapid rate. Here, a remarkably fast (∼30 s) and facile synthesis of crystalline quaternary chalcopyrite copper-zinc-iron-sulfide (CZIS) NCs is reported. These NCs show excellent catalytic properties by degrading a number of refractory organic dyes and converting nitro-compounds at a rapid rate. The valence and conduction band information of the newly designed NCs are extracted using scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy, which reveal energy levels suitable for performing redox chemistry by generating reactive radicals establishing NCs as efficient catalyst with multiple uses. Rapid synthesis of high quality phase-controlled CZIS NCs with robust catalytic activities could be useful for organic waste treatment.

Multidimensional self-assembly of peanut shaped PbS nanostructures

Peanut shaped PbS nanocrystals are synthesized and their self-assemblies have been studied by means of solvent evaporation. Multi-scale ordered self-assemblies of PbS nanopeanuts have been achieved by solvent evaporation route and interparticle interactions in the solution phase. The self-assembly mechanism produces compact monolayers with long-range ordering over macroscopic areas, which have been utilized for fabricating photovoltaics devices. A current–voltage response upon white light irradiation was measured with increasing photocurrent upon increasing incident light intensity. Repetitive on-off device response to white illumination flux is found to be sharp and repeatable over successive on/off irradiation cycles. Such simple self-assembled route is useful for the fabrication of technologically important ultra thin film materials for sensors and photovoltaic devices.

Efficient Solid-State Light-Emitting CuCdS Nanocrystals Synthesized in Air

Semiconductor nanocrystals (NCs) possess high photoluminescence (PL) typically in the solution phase. In contrary, PL rapidly quenches in the solid state. Efficient solid state luminescence can be achieved by inducing a large Stokes shift. Here we report on a novel synthesis of compositionally controlled CuCdS NCs in air avoiding the usual complexity of using inert atmosphere. These NCs show long-range color tunability over the entire visible range with a remarkable Stokes shift up to about 1.25 eV. Overcoating the NCs leads to a high solid-state PL quantum yield (QY) of ca. 55% measured by using an integrating sphere. Unique charge carrier recombination mechanisms have been recognized from the NCs, which are correlated to the internal NC structure probed by using extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS measurements show a Cu-rich surface and Cd-rich interior with 46% Cu(I) being randomly distributed within 84% of the NC volume creating additional transition states for PL. Color-tunable solid-state luminescence remains stable in air enabling fabrication of light-emitting diodes (LEDs).

Shape-controlled cobalt phosphide nanoparticles as volatile organic solvent sensor

Here we demonstrate shape-controlled cobalt phosphide nanostructures through a one-pot synthetic approach in a single step with precise control over the size, shape and composition. We found that the ratio of the cobalt precursor, phosphorus sources, capping ligands, and annealing time play a key role in determining the morphology of the cobalt phosphide nanostructures. For instance, a zero-to-one-dimensional nanostructure transformation was observed with an increase in annealing time. The aspect ratio of one-dimensional nanorods could flexibly be tuned by a subtle balance of cobalt precursor, phosphorous source and capping ligands. Furthermore, a rod-to-hollow sphere transformation could be observed upon a decrease in the phosphorous source. Of these nanostructures, branched nanowires showed an excellent volatile organic solvent vapor sensing performance selective to benzene compared to the linear chain hydrocarbon hexane. A novel yet simple synthesis strategy with wide varieties of a controlled morphology of cobalt phosphide is expected to open up a new avenue to design other metal phosphide nanostructures, which would offer superior applications in the field of energy harvesting and sensing.

Role of Acid–Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals

A binary ligand system composed of aliphatic carboxylic acids and primary amines of various chain lengths is commonly employed in diverse synthesis methods for CsPbBr3 nanocrystals (NCs). In this work, we have carried out a systematic study examining how the concentration of ligands (oleylamine and oleic acid) and the resulting acidity (or basicity) affects the hot-injection synthesis of CsPbBr3 NCs. We devise a general synthesis scheme for cesium lead bromide NCs which allows control over size, size distribution, shape, and phase (CsPbBr3 or Cs4PbBr6) by combining key insights on the acid–base interactions that rule this ligand system. Furthermore, our findings shed light upon the solubility of PbBr2 in this binary ligand system, and plausible mechanisms are suggested in order to understand the ligand-mediated phase control and structural stability of CsPbBr3 NCs.

Facile synthesis of composition and morphology modulated quaternary CuZnFeS colloidal nanocrystals for photovoltaic application

Quaternary semiconductor CuZnFeS (CZIS) nanocrystals (NCs) with controlled size, shape and composition composed of earth abundant elements have been successfully synthesized using the colloidal synthesis method. The size, shape and composition of the NCs have been controlled by tuning the reaction parameters to obtain NCs in the form of dots, triangles, hexagons, sheets, rods and wires. These quaternary CZIS NCs show high light absorbing properties towards visible to near infrared light with a high absorption coefficient suitable for photovoltaic applications. Utilizing layer-by-layer deposition of CIZS NCs films, heterojunction devices consisting of ITO/PEDOT : PSS/CZIS NCs/Al are fabricated for photovoltaic performance. The devices exhibit excellent rectification behavior (rectification ratio of ∼150) and good photoresponsitivity (on/off ratio of ∼55). The broad range of absorptions with strong extinction coefficient properties of CZIS NCs has been utilized to fabricate quantum dot sensitized solar cells (QDSSCs). Our synthesis protocol marks an advance for chalcopyrite NCs based solar cells and offers a possible template for the synthesis of other ternary and quaternary NCs with robust photoelectric properties.