Exciton Dynamics of Colloidal Semiconductor Quantum Well Stacks

Colloidal semiconductor nanoplatelets (NPLs) have recently emerged as a new class of colloidal nanocrystals. NPLs are quasi two-dimensional nanocrystals having atomically flat surfaces and have unique properties such as narrow photoluminescence (PL) emission (similar to 10 nm) and giant oscillator strength. NPLs can be self-assembled into stacks. These are one-dimensional superstructures that can contain tens or hundreds of NPLs in one chain

Nonradiative energy transfer in colloidal CdSe nanoplatelet films

Nonradiative energy transfer (NRET) has been extensively studied in colloidal nanocrystal (quantum dots) and nanorod (quantum wires) assemblies. In this work, we present the first account of spectroscopic evidence of NRET in solid thin films of CdSe based colloidal nanoplatelets (NPLs), also known as colloidal quantum wells. The NRET was investigated as a function of the concentration of two NPL populations with different vertical thicknesses via steady state and time resolved spectroscopy. NRET takes place from the NPLs with smaller vertical thickness (i.e., larger band gap) to the ones with a larger vertical thickness (i.e., smaller band gap) with efficiency up to similar to 60%. Here, we reveal that the NRET efficiency is limited in these NPL solid film assemblies due to the self-stacking of NPLs within their own population causing an increased distance between the donor-acceptor pairs, which is significantly different to previously studied colloidal quantum dot based architectures for nonradiative energy transfer

Orientation-Controlled Construction of Superstructures of Atomically-Flat Nanocrystals: Pushing the Limits of Ultra-Thin Colloidal Gain Media

We propose and demonstrate a method for the construction of highly uniform, multilayered, orientation-controlled superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using bi-phase liquid interface. These atomically-flat nanocrystals are sequentially deposited, all face-down onto a solid substrate, into slabs having monolayer-precise thickness and excellent homogeneity over several tens of cm2 areas. Owing to the near-unity surface coverage and film uniformity of this deposition technique, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin colloidal film having only 6 layers of NPLs, which corresponds to a mere 42 nm thickness. Furthermore, systematic studies of optical gain properties of these NPL superstructures constructed having precise numbers of NPL layers tuned from 6 to 15 revealed the reduction in the gain threshold with the increasing number of NPL monolayers, along with a continuous spectral shift in the position of the ASE peak (by ~18 nm). These observations can be well explained by the variation of the optical field confinement factor with the NPL waveguide thickness and propagation wavelength. This work demonstrates the possibility of fabricating thickness-tunable, large-area three-dimensional superstructures made of NPL building blocks, which can be additively constructed one monolayer at a time. The proposed technique can also be extended to build hybrid NPL films of mixed orientations and allow for precise large-area device engineering.Comment: 23 pages, 4 figure

Ultralow Threshold One-Photon- and Two-Photon-Pumped Optical Gain Media of Blue-Emitting Colloidal Quantum Dot Films

Colloidal quantum dots (QDs) offer advantageous properties as an optical gain media for lasers. Optical gain in the QDs has been shown in the whole visible spectrum, yet it has been intrinsically challenging to realize efficient amplified spontaneous emission (ASE) and lasing in the blue region of the visible spectrum. Here, we synthesize large-sized core/gradient shell CdZnS/ZnS QDs as an efficient optical gain media in the blue spectral range. In this Letter, we demonstrate for the first time that two-photon-absorption-pumped ASE from the blue-emitting QD is achievable with a threshold as low as 6 mJ/cm(2). Utilizing these QDs, we also report one-photon-absorption-pumped ASE at an ultralow threshold of similar to 60 mu J/cm(2), which is comparable to the state-of-the-art red-emitting QD-based gain media. This one-photon-pumped ASE threshold is an order of magnitude better than that of the previously reported best blue-emitting QD-based gain media

Light-Induced Currents at Domain Walls in Multiferroic BiFeO3.

Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters

CdSe/CdSe1-xTex Core/Crown Heteronanoplatelets: Tuning the Excitonic Properties without Changing the Thickness

Here we designed and synthesized CdSe/CdSe1-xTex core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe1-xTex crown region from pure CdTe (x = 1.00) to almost pure CdSe doped with several Te atoms (x = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of similar to 570 nm (for CdSe1-xTex crown with x = 0.02) and similar to 660 nm (for CdSe1-xTex crown with x = 1.00). Furthermore, with the formation of the CdSe1-xTex crown region, we observed substantially improved photoluminescence quantum yields (up to similar to 95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from similar to 49 up to similar to 326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and-harvesting applications

Attractive versus Repulsive Excitonic Interactions of Colloidal Quantum Dots Control Blue- to Red-Shifting (and Non-shifting) Amplified Spontaneous Emission

Tunable, high-performance, two-photon absorption (TPA)-based amplified spontaneous emission (ASE) from near-unity quantum efficiency colloidal quantum dots (CQDs) is reported. Besides the absolute spectral tuning of ASE, the relative spectral tuning of ASE peak with respect to spontaneous emission was shown through engineering excitonic interactions in quasi-type-II CdSe/CdS core/shell CQDs. With core shell size adjustments, it was revealed that Coulombic exciton-exciton interactions can be tuned to be attractive (type-I-like) or repulsive (type-II-like) leading to red- or blue-shifted ASE peak, respectively, and that nonshifting ASE can be achieved with the right core shell combinations. The possibility of obtaining ASE at a specific wavelength from both type-I-like and type-II-like CQDs was also demonstrated. The experimental observations were supported by parametric quantum-mechanical modeling, shedding light on the type-tunability. These excitonically engineered CQD-solids exhibited TPA-based ASE threshold as low as 6.5 mJ/cm(2) under 800 nm excitation, displaying one of the highest values of TPA cross-section of 44 660 GM

Ultralow-threshold up-converted lasing in oligofluorenes with tailored strong nonlinear absorption

Nonlinear optical response in organic semiconductors has been an attractive property for many practical applications. For frequency up-converter lasers, to date, conjugated polymers, fluorescent dyes and small organic molecules have been proposed but their performances have been severely limited due to the difficulty of simultaneously achieving strong nonlinear optical response and high performance optical gain. In this work, we show that structurally designed truxene-based star-shaped oligofluorenes exhibit strong structure-property relationships enabling enhanced nonlinear optical response with favorable optical gain performance. As the number of fluorene repeat units in each arm is increased from 3 to 6, these molecules demonstrate a two-photon absorption cross-section as high as 2200 GM, which is comparable to that of linear conjugated polymers. Tailored truxene oligomers with six fluorene units in each arm (T6) show two-photon absorption pumped amplified spontaneous emission with a threshold as low as 2.43 mJ/cm2, which is better than that of the lowest reported threshold in organic semiconductors. Furthermore, we show a frequency up-converted laser using the newly designed and synthesized star-shaped oligomer T6 with a threshold as low as 3.1 mJ/cm2, which is more than an order of magnitude lower than that of any conjugated polymer. Thus, these oligomers with enhanced nonlinear optical properties are highly attractive for bio-integrated applications such as photodynamic therapy and in-vivo bio-sensing

Continuously Tunable Emission in Inverted Type-I CdS/CdSe Core/Crown Semiconductor Nanoplatelets

The synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdSe heteronanoplatelets are grown with thickness of four monolayers using seed-mediated method. In this work, it is shown that the emission peak of the resulting CdS/CdSe heteronanoplatelets can be continuously spectrally tuned between the peak emission wavelengths of the core only CdS nanoplatelets (421 nm) and CdSe nanoplatelets (515 nm) having the same vertical thickness. In these inverted Type-I nanoplatelets, the unique continuous tunable emission is enabled by adjusting the lateral width of the CdSe crown, having a narrower bandgap, around the core CdS nanoplatelet, having a wider bandgap, as a result of the controlled lateral quantum confinement in the crown region additional to the pure vertical confinement. As a proof-of-concept demonstration, a white light generation is shown by using color conversion with these CdS/CdSe heteronanoplatelets having finely tuned thin crowns, resulting in a color rendering index of 80. The robust control of the electronic structure in such inverted Type-I heteronanoplatelets achieved by tailoring the lateral extent of the crown coating around the core template presents a new enabling pathway for bandgap engineering in solution-processed quantum wells