Vikram C. Sundar

Biological applications of quantum dots

Quantum dots (QDs) are a novel class of inorganic fluorophore which are gaining widespread recognition as a result of their exceptional photophysical properties. They are rapidly being applied to existing and emerging technologies, and could have an important role in many areas. Significant challenges remain, however, which must be understood and more fully defined before they can be widely validated. This review provides on overview of QD technology, covering QD characteristics, synthesis methods, and the applications in which they have been put to use. The influence of synthesis methods on QD characteristics and their subsequent suitability to different applications is discussed, and a broad outline of the technologies into which they have been incorporated is presented, and the relative merits and weaknesses of their incorporation are evaluated. The potential for further development, and inclusion in other technologies is also discussed, and barriers restricting further progress specified, particularly with regard to the poorly understood surface chemistry of QDs, the potential for alteration of function of biological molecules when complexed with QDs, and on a larger scale the significant potential for cytotoxicity both in vitro and in vivo.

Quantum-dot optical temperature probes

The steady-state photoluminescence (PL) properties of cadmium selenide quantum dots (QDs) with a zinc sulfide overlayer [(CdSe)ZnS] can be strongly dependent on temperature in the range from 100 to 315 K. The PL intensity from 50 to 55 A (CdSe)ZnS QDs in poly(lauryl methacrylate) matrices increases by a factor of ∼5 when the temperature is decreased from 315 to 100 K, and the peak of the emission band is blueshifted by 20 nm over the same range. The change in PL intensity is appreciable, linear, and reversible (−1.3% per °C) for temperatures close to ambient conditions. These properties of (CdSe)ZnS dots are retained in a variety of matrices including polymer and sol–gel films, and they are independent of excitation wavelength above the band gap. The significant temperature dependence of the luminescence combined with its insensitivity to oxygen quenching establishes (CdSe)ZnS dots as optical temperature indicators for temperature-sensitive coatings.

Color-selective semiconductor nanocrystal laser

Theoretical predictions of the benefits of three-dimensional quantum confinement have provided motivation for the development of quantum-dot lasers. Such lasers, developed in the case of self-assembled quantum dots, have not been successfully demonstrated with quantum-confined colloidal nanocrystals (NCs). Here, using recently developed NC-titania chemistry, we report the successful development of an optically pumped, NC-based distributed feedback laser, in which the narrow gain profiles of these nanoparticles have been matched with the feedback of a second-order distributed feedback laser. This laser, whose output color can be selected by choosing appropriately sized nanocrystals, operates at 80 K and at room temperature.

Bioconjugation of Highly Luminescent Colloidal CdSe–ZnS Quantum Dots with an Engineered Two-Domain Recombinant Protein

We present a novel approach, based on molecular self-assembly driven by electrostatic attractions, for conjugating inorganic colloidal semiconductor nanocrystals (quantum dots: QDs) having negatively charged surfaces with a two-domain recombinant protein bearing a positively charged C-terminal leucine zipper domain. Aggregation-free QD/protein conjugate dispersions were prepared. Conjugates retain both properties of the starting materials, i.e., biological activity of the protein and spectroscopic characteristics of the QDs. Such hybrid bio-inorganic conjugates represent a powerful fluorescent tracking tool, because they combine advantages of CdSe–ZnS quantum dots, such as chemical stability and a wide range of size-dependent luminescence emission properties, with a straightforward electrostatic conjugation approach. We describe the design and preparation of a model QD/protein conjugate and present functional characterization of the conjugate using luminescence and bioassays.

Fabrication and optical properties of polymeric waveguides containing nanocrystalline quantum dots

We report on the fabrication of polymer waveguides containing infrared-emitting nanocrystal quantum dots. Both PbSe and InAs nanocrystal quantum dots are incorporated into a fluorinated polymer by a surface functionalization method. The optical properties of the nanocrystal quantum dots are shown to be unaffected by the entire fabrication process. This method may provide a versatile platform for integration of nanocrystal quantum dots into planar photonic circuits.

Field-effect transistors made from macroscopic single crystals of tetracene and related semiconductors on polymer dielectrics

We report properties of devices made by the adhesion of semiconductor crystals, including several tetracene specimens, to polymer gate dielectrics along with measurements of tetracene crystals on conventional Si/SiO 2 dielectric surfaces. For the best tetracene, pentacene, and alpha-6T devices, mobilities exceeding 0.1 cm 2 /V were measured, correlating well with expectations based on the literature, and in the case of tetracene and alpha-6T, exceeding the thin film mobility value. The devices were prepared in the open laboratory using simpler crystal handling techniques than had been thought necessary.

Narrow features in metals at the interfaces between different etch resists

The ability to create structures on length scales below 100 nm easily is a challenging feat. We report here a facile technique for the fabrication of such structures in gold (Au) with feature sizes smaller than 50 nm, utilizing two families of Au etch resists in conjunction. The first resist family consists of self-assembled monolayers (SAMs) of alkane thiols on Au, which provide substantial resistance against cyanide etch solutions. The second class consists of metals deposited on the surface of Au, which also provide similar resistance of the Au film to CN etchants but are not conducive for the formation of SAMs. Selective etching is initiated at the interface between these resists, proceeds into the Au layer, and results in narrow trenches in the Au film. Our protocol allows for the sequential removal of both resists and thus permits the creation of planar Au surfaces with well-defined sub-50-nm etch patterns.

Making waveguides containing nanocrystalline quantum dots

A new material platform is described that enables inclusion of nanocrystalline quantum dots into a polymer. This technology is compatible with semiconductor processing and may enable integration of active materials into current waveguide technologies. We describe the steps preformed to fabricate a waveguide chip that contains IR-emitting quantum dots. Optical tests demonstrate guiding and preservation of the quantum dots optical properties through the processing steps. Time resolved optical measurements indicate presence of gain in the InAs quantum dot impregnated polymer.

Structural and Initial Optical Characterization of Basalia Spicules in the Glass Sponge Euplectella

We report on the structural properties of siliceous spicules, found in the hexactinellid sponge Euplectella . Selective chemical etching of Euplectella spicules in bleach reveals the presence of a striated arrangement of silica spheres (∼50-200 nm in diameter) placed radially around a central, high-organic content cylindrical region. Complementary etching experiments with HF indicate a graded variation of silica content in these shells. The presence of such a multi-layered and variable-composition structure affords these spicules not only with enhanced structural integrity but could also be reflected in their optical properties. Qualitative, index-matched, refractive index measurements are shown to support this expectation.