Semiconductor nanocrystals exhibit size-tunable absorption and emission ranging from the ultraviolet (UV) to the near-infrared (NIR) spectral range, high absorption coefficient, and high photoluminescence quantum yield. Effective surface passivation of these so-called quantum dots (QDs) may be achieved by growing a shell of another semiconductor material. The resulting core/shell QDs can be considered as a model system to study and optimize structure/property relations. A special case consists in growing thick shells (1.5 up to few tens of nanometers) to produce “giant” QDs (g-QDs). Tailoring the chemical composition and structure of CdSe/CdS and PbS/CdS g-QDs is a promising approach to widen the spectral separation of absorption and emission spectra (i.e., the Stokes shift), improve the isolation of photogenerated carriers from surface defects and enhance charge carrier lifetime and mobility. However, most stable systems are limited by a thick CdS shell, which strongly absorbs radiation below 500 nm, covering the UV and part of the visible range. Modification of the interfacial region between the core and shell of g-QDs or tuning their doping with narrow band gap semiconductors are effective approaches to circumvent this challenge. In addition, the synthesis of g-QDs composed of environmentally friendly elements (e.g., CuInSe2/CuInS2) represents an alternative to extend their absorption into the NIR range. Additionally, the band gap and band alignment of g-QDs can be engineered by proper selection of the constituents according to their band edge positions and by tuning their stoichiometry during wet chemical synthesis. In most cases, the quasi-type II localization regime of electrons and holes is achieved. In this type of g-QDs, electrons can leak into the shell region, while the holes remain confined within the core region. This electron–hole spatial distribution is advantageous for optoelectronic devices, resulting in efficient electron–hole separation while maintaining good stability.
This Account provides an overview of emerging engineering strategies that can be adopted to optimize structure/property relations in colloidal g-QDs for efficient photon management or charge separation/transfer. In particular, we focus on our recent contributions to this rapidly expanding field of research. We summarize the design and synthesis of a variety of colloidal g-QDs with the aim of tuning the optical properties, such as absorption/emission in a wide region of the solar spectrum, which allows enlargement of their Stokes shift. We also describe the band alignment within these systems, charge carrier dynamics, and charge transfer from g-QDs into semiconducting oxides. We show how these tailored g-QDs may be used as active components in luminescent solar concentrators, photoelectrochemical cells for hydrogen generation, QD-sensitized solar cells and optical nanothermometers. In each case, we aim at providing insights on structure/property relationships and on how to optimize them toward improving device performance. Finally, we describe perspectives for future work, sketching new directions and opportunities in this field of research at the intersection between chemistry, physics, materials science and engineering.
InPhotonics offers versatile Raman products for laboratory, plant, and field use. The following list of publications and presentations demonstrates the versatility and performance features of our spectrometers and fiber optic probes. Contact us to discuss our recent application successes.
Journal Publications and Proceedings
- A Nano-engineered Sensor to Detect Vibrational Modes of Warfare Agents/Explosives Using Surface-enhanced Raman Scattering, Proc. SPIE Vol. 5403, in press.
- High Resolution UV Echelle Spectrograph for Environmental Sensing, Proc. SPIE Vol. 5269, 34-41(2004).
- Surface-Enhanced Raman Spectroscopy for Homeland Defense, Proc. SPIE Vol. 5269, 1-8 (2004).
- Surface-Enhanced Raman for Monitoring Toxins in Water, Proc. SPIE Vol. 5268, 340 -348 (2004).
- SERS of Whole Cell Bacteria and Trace Levels of Biological Molecules, Proc. SPIE, Vol. 4577, 182-92 (2002).
- SERS detection of the nuclear weapons explosive triaminotrinitrobenzene, Proc. SPIE, Vol. 4577, 230 -238 (2002).
- Surface-Enhanced Raman as a Water Monitor for Warfare Agents, Proc. SPIE, Vol. 4577, 158-65 (2002).
- Field Tests of Cone Penetrometer-based Raman Spectroscopy DNAPL Characterization System, submitted 2000.
- Surface-Enhanced Raman Detection of 2,4-Dinitrotoluene Impurity Vapor as a Marker to Locate Landmines, Anal. Chem., 72, 5834 (2000).
- Chemical Identification with a Portable Raman Analyzer and Forensic Spectral Database, Spectroscopy, 15(10), 32 (2000).
- Nonintrusive Analysis of Chemical Agent Identification Sets Using a Portable Fiber-Optic Raman Spectrometer, Appl. Spectrosc., 53, 850 (1999).
- Advances in Landmine Detection using Surface-Enhanced Raman Spectroscopy, Aerosense '99, 3710, Abstract 35 (1999).
- Surface Enhanced Raman Sensor for Nitroexplosive Vapor, Aerosense '98, 3392, 469 (1998).
- Analytical Chiral Purity Verification using Raman Optical Activity, Appl. Spectrosc., 50, 681 (1996).
- Echelle Spectroscopy and CCD's, an Ideal Union for Fiber Optic Raman Systems, Proceeding of the Third International Conference on Scientific Optical Imaging, Georgetown, Grand Cayman Island British West Indies, Edited by B. Denton, Royal Chemistry Society, November, 1995.
Simple Approaches to Optical Sensing in the Subsurface Environment, Environmental Monitoring and Hazardous Waste Site Remediation, Proc. SPIE, 2504, 52 (1995).
- The Utilization of Diode Lasers for Raman Spectroscopy, Spectrochimica Acta Part A, 51, 1779 (1995)
- Nonaqueous Phase Liquids: Searching for the Needle in a Haystack Proc. Symposium on Field Screening Methods for Hazardous Wastes and Toxic Chemicals, Las Vegas, p. 443 (1995).
- Fiber Optic Spectroelectrochemical Sensing of Trichloroethylene, Proc. Symposium on Field Screening Methods for Hazardous Wastes and Toxic Chemicals, Las Vegas, p. 204 (1995).
- Analytical Chiral Purity Verification Using Raman Optical Activity, Appl. Spectroscopy, 50, 681 (1995).
- Fiber Optic Raman Chemical Sensors, in Chemical Sensors II, M. Butler et al., eds. (The Electrochemical Society, Pennington, N.J., 1993).
- Spectroelectrochemical Technologies and Instrumentation for Environmental and Process Monitoring, SPIE OE/LASE92 - Laser Spectroscopy , Vol. 1637, 82 (1992).
- Compact Raman Instrumentation for Process and Environmental Monitoring, SPIE OE/LASE91 - Laser Spectroscopy , Vol. 1434, 127 (1991).
- The Utilization of Holographic Bragg Diffraction Filters for Rayleigh Line Rejection in Raman Spectroscopy, Applied Spectroscopy, 44, 1558 (1990).
- The Prospect of Utilizing Surface Enhanced Raman Spectroscopy (SERS) for Bio- and Biomedical Sensing, SPIE Biomedical Optics 90, Vol. 1201, Invited Paper pg. 438 (1990).
- The Suitability of Surface Enhanced Raman Spectroscopy (SERS) to Fiber Optic Chemical Sensing of Aromatic Hydrocarbon Contamination in Groundwater, Proceedings of the First International Symposium on Field Screening Methods for Hazardous Waste Sites Investigations, EPA, 33, 1988.
- Surface Enhanced Raman Spectroscopy (SERS), an Existing or Emerging Chemical Sensing Technology?, Proceedings of the Department of Energy In Situ Characterization and Monitoring Technologies Workshop, DOE/HWP-62, 1988.
- Feasibility Studies for the Detection of Organic Surface and Subsurface Water Contaminants by Surface-Enhanced Raman Spectroscopy on Silver Electrodes, Anal. Chem., 59, 2559 (1987).
Recent Conference Presentations
- Fiber Optic Probes for Process Monitoring Applications, 16th International Forum on Process Analytical Chemistry, San Diego, CA, January 22 - 25, 2002.
- SERS Detection of the Nuclear Weapons Explosive Triaminotrinitrobenzene, Photonics Boston, Boston, MA, October 28 - November 2, 2001.
- Raman Analysis of Corrosion Anions in High-level Waste, Photonics Boston, Boston, MA, October 28 - November 2, 2001.
- SERS of Whole Cell Bacteria and Trace Levels of Biological Molecules, Photonics Boston, Boston, MA, October 28 - November 2, 2001.
- Surface-enhanced Raman as a Water Monitor for Warfare Agents, Photonics Boston, Boston, MA, October 28 - November 2, 2001.
- Advances in Fiber Optic Probes: Raman Measurements in Nasty Places, 28th FACSS Meeting, October 7-12, 2001.
- Raman Spectroscopy to Monitor the Quality of Turbine Oil, Gulf Coast Conference 2001, Galveston Island, TX, September 2001.
- Development of a Fiber Optic Raman Probe for Hydrothermal Reactor Monitoring , Abstract 1992, Pittcon 2001, New Orleans, LA, March 2001.
- Automated Spectral Interpretation of Surface Enhanced Raman Spectra Collected from Landmine Vapor Signatures, Abstract 1069, Pittcon 2001, New Orleans, LA, March 2001.
- Raman Library Searching: A Statistical Approach to the Effects of Instrument Miscalibration, Resolution and Spectral Pretreatment, Abstract 1063, Pittcon 2001, New Orleans, LA, March 2001.
- Principal Component Analysis Applied to Surface Enhanced Raman Spectra of Oligionucleotides and their Bases, Abstract 451, Pittcon 2001, New Orleans, LA, March 2001.
- "Field Screening and Monitoring Applications of Raman Spectroscopy," Abstract 508, PittCon '99, Orlando, FL, March 1999.
- "The Design of Fiber Optic Raman Probes for Unique Applications," Abstract 514, PittCon '99, Orlando, FL, March 1999.
- "Environmental Applications of Raman Fiber Optic Sensors," Abstract 66, 25th FACSS Meeting, Austin, Texas, October 11 - 15, 1998.
- "Chemical Sensor," U.S. Patent 4,892,834 (January, 1990).
- "Modified Electrically Conductive Polymers", US 4,933,394 (June, 1990).
- "Assaying for a Biologically Active Component", US 4,997,526 (March, 1991).
- "Apparatus for Measuring Raman Spectra Over Optical Fibers", US 5,112,127 (1992).
- "Substrate and Apparatus for Surface Enhanced Raman Spectroscopy", US 5,255,067 (October, 1993).
- "Dual Function Safety and Calibration Accessory for Raman and Other Spectroscopic Sampling", pending.
Application Notes from our Research Affiliate (requires Adobe® Acrobat Reader)
- Detection of DNAPLs by Raman Spectroscopy
- Buried Landmine Detection with SERS
- A Raman Fiber Optic Probe to Monitor Hydrothermal Oxidation Processes
- In-situ Monitoring of Nuclear Waste
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