Dmitriy S. Dolzhnikov

Low-Threshold Stimulated Emission Using Colloidal Quantum Wells

The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/cm(2) and gain as high as 600 cm(-1), both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.

Composition-matched molecular “solders” for semiconductors

Soldering semiconductor nanoparticles The optical and electronic properties of semiconductor nanoparticles can be tuned through changes in their size and composition. However, poor contact between interfaces can degrade nanoparticle performance in devices. Dolzhnikov et al. report the synthesis of a gel-like “solder” for metal chalcogenide nanoparticles, such as cadmium selenide and lead telluride, by cross-linking molecular wires of these materials. Science, this issue p. 425 A gel material mechanically bonds semiconductor nanoparticles and improves their electronic properties. We propose a general strategy to synthesize largely unexplored soluble chalcogenidometallates of cadmium, lead, and bismuth. These compounds can be used as “solders” for semiconductors widely used in photovoltaics and thermoelectrics. The addition of solder helped to bond crystal surfaces and link nano- or mesoscale particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder was used as a soluble precursor for CdSe films with electron mobilities exceeding 300 square centimeters per volt-second. CdTe, PbTe, and Bi2Te3 powders were molded into various shapes in the presence of a small additive of composition-matched chalcogenidometallate or chalcogel, thus opening new design spaces for semiconductor technologies.

Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

Carrier Cooling in Colloidal Quantum Wells

It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators.

Surface Functionalization of Semiconductor and Oxide Nanocrystals with Small Inorganic Oxoanions (PO43–, MoO42–) and Polyoxometalate Ligands

In this work, we study the functionalization of the nanocrystal (NC) surface with inorganic oxo ligands, which bring a new set of functionalities to all-inorganic colloidal nanomaterials. We show that simple inorganic oxoanions, such as PO4(3-) and MoO4(2-), exhibit strong binding affinity to the surface of various II-VI and III-V semiconductor and metal oxide NCs. ζ-Potential titration offered a useful tool to differentiate the binding affinities of inorganic ligands toward different NCs. Direct comparison of the binding affinity of oxo and chalcogenidometallate ligands revealed that the former ligands form a stronger bond with oxide NCs (e.g., Fe2O3, ZnO, and TiO2), while the latter prefer binding to metal chalcogenide NCs (e.g., CdSe). The binding between NCs and oxo ligands strengthens when moving from small oxoanions to polyoxometallates (POMs). We also show that small oxo ligands and POMs make it possible to tailor NC properties. For example, we observed improved stability upon Li(+)-ion intercalation into the films of Fe2O3 hollow NCs when capped with MoO4(2-) ligands. We also observed lower overpotential and enhanced exchange current density for water oxidation using Fe2O3 NCs capped with [P2Mo18O62](6-) ligands and even more so for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] with POM as the capping ligand.

Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular "Solders": Approaching Single Crystal Mobility.

Crystalline silicon-based complementary metal-oxide-semiconductor transistors have become a dominant platform for todays electronics. For such devices, expensive and complicated vacuum processes are used in the preparation of active layers. This increases cost and restricts the scope of applications. Here, we demonstrate high-performance solution-processed CdSe nanocrystal (NC) field-effect transistors (FETs) that exhibit very high carrier mobilities (over 400 cm(2)/(V s)). This is comparable to the carrier mobilities of crystalline silicon-based transistors. Furthermore, our NC FETs exhibit high operational stability and MHz switching speeds. These NC FETs are prepared by spin coating colloidal solutions of CdSe NCs capped with molecular solders [Cd2Se3](2-) onto various oxide gate dielectrics followed by thermal annealing. We show that the nature of gate dielectrics plays an important role in soldered CdSe NC FETs. The capacitance of dielectrics and the NC electronic structure near gate dielectric affect the distribution of localized traps and trap filling, determining carrier mobility and operational stability of the NC FETs. We expand the application of the NC soldering process to core-shell NCs consisting of a III-V InAs core and a CdSe shell with composition-matched [Cd2Se3](2-) molecular solders. Soldering CdSe shells forms nanoheterostructured material that combines high electron mobility and near-IR photoresponse.

Two-dimensional electronic spectroscopy of CdSe nanoparticles at very low pulse power

Nanoparticles have been proposed as a promising material for creating devices that harvest, transport, and manipulate energy and electrons. Ultrafast charge carrier dynamics represent a critical design aspect and are dependent on both size and shape of the nanoparticle. Spectroscopic investigation of the electronic structure and dynamics of these systems is complicated by sample inhomogeneity, which broadens peaks and leads to ambiguity in interpretation of both spectra and dynamics. Here, we use two-dimensional electronic spectroscopy to remove inhomogeneous broadening and to clarify interpretation of measured dynamics. We specifically investigate the effect of nanoparticle shape on the electronic structure and ultrafast electronic dynamics in the band-edge exciton states of CdSe quantum dots, nanorods, and nanoplatelets. Particle size was chosen to enable straightforward comparisons of the effects of particle shape on the spectra and dynamics without retuning the laser source. The spectra were measured with low pulse powers (generally <1 nJ/pulse), using short pulses (~12 fs) to minimize interference from solvent contributions to the spectra, ambiguities in the dynamics due to pulse-overlap effects, and contributions to the dynamics from multi-exciton effects. The lowest two exciton states are clearly resolved in spectra of quantum dots but unresolved for nanorods and nanoplates, in agreement with previous spectroscopic and theoretical results. In all nanoparticles, ultrafast dynamics measurements show strong evidence of electronic relaxation into the lowest energy exciton state within ~30 fs, a timescale not observable in previous dynamics measurements of similar systems. These dynamics are unambiguously assigned to hole relaxation, as the higher lying electronic excited states are not energetically accessible in these experiments. Clear evidence of coherent superpositions of the lowest two exciton states were not seen in any of the particles studied, in contrast to recent results from work on quantum dots.

Dispersion-free continuum two-dimensional electronic spectrometer

Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an ~180  nm window of the available bandwidth at ~10  fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll a.

Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy

Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te2– Ligands Lead To Charge Carrier Diffusion Lengths Over 2 μm

We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm(2)/(V s)). Our TRMC findings show that Te(2-) capped CdTe NCs show a marked improvement in carrier mobility (11 cm(2)/(V s)), indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films.