Patricia L. Nichols

Contact printing of compositionally graded CdS(x)Se(1-x) nanowire parallel arrays for tunable photodetectors.

Spatially composition-graded CdS(x)Se(1-x) (x = 0-1) nanowires are grown and transferred as parallel arrays onto Si/SiO(2) substrates by a one-step, directional contact printing process. Upon subsequent device fabrication, an array of tunable-wavelength photodetectors is demonstrated. From the spectral photoconductivity measurements, the cutoff wavelength for the device array, as determined by the bandgap, is shown to cover a significant portion of the visible spectrum. The ability to transfer a collection of crystalline semiconductor nanowires while preserving the spatially graded composition may enable a wide range of applications, such as tunable lasers and photodetectors, efficient photovoltaics, and multiplexed chemical sensors.

Influence of supersaturation and spontaneous catalyst formation on the growth of PbS wires: toward a unified understanding of growth modes.

High quality stoichiometric lead sulfide (PbS) wires were synthesized by a simple chemical vapor deposition (CVD) process using pure PbS powder as the material source. Growth mechanisms were systematically investigated under various growth conditions, with three modes of growth identified: direct vapor-liquid-solid (VLS) wire growth nucleating from the substrate surface, bulk PbS crystallites by vapor-solid (VS) deposition, and subsequent VLS growth nucleating on top of the bulk deposition through spontaneously formed catalyst particles. Furthermore, we found that these growth modes can be organized in terms of different levels of supersaturation, with VS bulk deposition dominating at high supersaturation and VLS wire growth on the substrate dominating at low supersaturation. At intermediate supersaturation, the bulk VS deposition can form larger crystallites with domains of similarly oriented wires extending from the flat facets. Both predeposited catalysts and spontaneously formed Pb particles were observed as nucleation catalysts, and their interplay leads to various interesting growth scenarios such as reversely tapered growth with increasing diameter. The VLS growth mechanism was confirmed by the presence of Pb-rich caps revealed in an elaborate cross-sectional transmission electron microscopy (TEM) experiment after focused ion beam milling in a modified lift-out procedure. Temperature-dependent photoluminescence (PL) of PbS wires was performed in the mid-infrared wavelength range for the first time, demonstrating strong light emission from band edge, blue-shifted with increasing temperature. The high optical quality of PbS wires may lead to important applications in mid-infrared photonics. The substrate growth temperature as low as 400 °C allows for silicon-compatible processing for integrated optoelectronics applications.

Simultaneous two-color lasing in a single CdSSe heterostructure nanosheet

The ability of a single monolithic semiconductor structure to emit or lase in a broad spectrum range is of great importance for many applications such as solid-state lighting and multi-spectrum detection. But spectral range of a laser or light-emitting diode made of a given semiconductor is typically limited by its emission or gain bandwidth. Due to lattice mismatch, it is typically difficult to grow thin film or bulk materials with very different bandgaps in a monolithic fashion. But nanomaterials such as nanowires, nanobelts, nanosheets provide a unique opportunity. Here we report our experimental results demonstrating simultaneous lasing in two visible colors at 526 and 623 nm from a single CdSSe heterostructure nanosheet at room temperature. The 97 nm wavelength separation of the two colors is significantly larger than the gain bandwidth of a typical single II‐VI semiconductor material. Such lasing and light emission in a wide spectrum range from a single monolithic structure will have important applications mentioned above. (Some figures may appear in colour only in the online journal)

CdxPb1–xS Alloy Nanowires and Heterostructures with Simultaneous Emission in Mid-Infrared and Visible Wavelengths

Alloying of CdS and PbS could potentially provide an important semiconductor with a wide range of bandgaps, with bandedge emission from mid-infrared to visible green, for various optoelectronic applications. We investigate the possibility of CdPbS alloy formation in nanowire and nanobelt forms, especially the dependence of alloy composition on two different cooling routes. Our results show that rapid cooling immediately after the growth phase can lead to a high-quality uniform alloy with Cd composition larger than possible at thermal equilibrium and by natural cooling. On the contrary, unassisted natural cooling leads to the formation of axial or core-shell heterostructures, containing segments with pure CdS and CdPbS alloys with lower Cd content than through rapid cooling. Such heterostructures with green and mid-infrared emission provide simultaneous access to two widely separated wavelengths from a single monolithic structure and can be important for many applications. Our results can help identify strategies for growing nanostructures with uniform alloy of high Cd incorporation, core-shell structures with shell serving as a passivating or protecting layer, or interesting longitudinal heterostructures. Both various heterostructures and uniform alloys of these materials could be important for high-efficiency solar cells, novel detectors, and nanolasing in wide spectral ranges.

Single-crystal erbium chloride silicate nanowires as a Si-compatible light emission material in communication wavelength

We report on the first synthesis and structural characterizations of a new Erbium (Er) compound, the erbium chloride silicate (ECS, Er3Cl(SiO4)2) single crystal in a Si-ECS core-shell nanowire form. The Er-concentration in ECS at 1.6x1022 cm−3 is three orders of magnitude higher than that of the Er-doped materials. Photoluminescence spectra at both low and room temperatures exhibit well separated sharp emission lines in the near infrared region. The new single-crystal erbium-compound nanowires provide a unique Si-compatible material for high-gain light emission in communication wavelength and for many other photonic applications.

Long lifetime, high density single-crystal erbium compound nanowires as a high optical gain material

Erbium-containing materials of long lifetime and high Er density are important for achieving strong luminescence and high optical gain in compact integrated photonics devices. We have systematically studied the lifetime and crystal quality as a function of growth conditions for an erbium compound that we recently reported, erbium chloride silicate (ECS). The lifetime for the best quality ECS nanowires can be as long as 540 μs, the longest for high-density Er-materials, representing a lifetime-density product as high as 8.7 × 1018 s cm−3. Such high density, long lifetime erbium materials can find many interesting applications such as compact lasers or amplifiers.

Semiconductor Alloy Nanowires and Nanobelts With Tunable Optical Properties

Recent advancements in the study of alloy semiconductor nanowires (NWs) and nanobelts are reviewed with a special perspective on their applications in optoelectronics with widely tunable bandgaps. Special emphasis is on the composition-graded alloy NWs. An extremely wide range of alloy compositions (thus bandgaps) can be achieved on a single substrate in a single growth run, creating an unprecedented materials capability that is not possible with planar epitaxial growth. Applications of such unique materials in widely tunable lasers and full-spectrum solar cells are discussed.

Mid-infrared lasing in a single lead sulfide subwavelength wire at 180 K

We report mid-infrared lasing around 3 μm from a single PbS subwavelength wire, with a cavity volume less than the wavelength cubed at 0.44 λ³. The maximal lasing temperature is 180 K under pulse operation.

Simultaneous green and red lasing in a single CdSSe heterostructure nanosheet at room temperature

We report simultaneous lasing in green and red colors at room temperature from a single CdSSe heterostructure nanosheet. The 51 nm wavelength separation exceeds the typical gain bandwidth of a single composition material.

Single-crystal erbium chloride silicate nanowires as a novel Si-compatible source at 1.53 µm

We report on synthesis of single crystal erbium chloride silicate nanowires, which exhibit linear power dependence of 1.53 µm band upto high pumping power, providing a new Si-compatible material for realizing 1.53 µm lasers.