Burda Research Group

Research

Topics:

Nanoscience:

Chemical Dynamics at the Nanostructure Surface:

Femtosecond Pump-Probe experiments in the visible-IR range are currently employed to investigate the chemical dynamics that leads to the observed photochemistry on crystalline and amorphous solid-liquid interfaces. The relaxation dynamics and surface temperatures as function of time are studied by laser up-conversion techniques. Particularly interesting are cases where interesting biomolecular probes are attached to inorganic surfaces, forming an interface between inorganic and biological material. Thereby the flux of charge, energy, and heat can be studied, quantified, and controlled on the molecular level. Interface processes on our novel nanomaterials are currently of pivotal interest in our group.

Photocatalysis using Nanoscale Materials:

Since the discovery of its photocatalytic activity in the early 1970's, interest in the technology of TiO₂ photocatalysis has progressed rapidly over the past ten years. However, widespread technological use of TiO₂ is still impaired by the fact that ultraviolet light, which is required for its activation, constitutes only a small fraction of the solar spectrum. Any improvement of the photocatalytic efficiency of TiO₂, by shifting its optical response to the visible range, will have a profound positive effect. In collaboration with Prof. James Gole (GaTech) research is underway to advance the development, characterization, and fundamental understanding of a novel class of visible-light activated photocatalysts based on TiO₂ nanoparticles.

Photovoltaics with Quantum Dot Assemblies:

One of the most exciting areas where nanotechnology can have a broad impact on our future is photovoltaics. Conventional photovoltaic devices suffer from problems related to extended defects and do not possess the tunability that can be achieve using quantum confined nanomaterials. We explore the feasibility of this approach with ternary nanomaterials for photovoltaic devices: (CuInSe2 and related compounds). By controlled chemistry we tune the bandgap of the material so that it matches well with the solar spectrum and render the nanoparticles ideal for photovoltaic applications.

Bio-imaging with Nanoparticles:

The advantages of semiconductor nanocrystals for optical imaging are adjustable emission wavelength, high emission efficiency, good photostability, broad excitation range, and long emission lifetimes. Due to their enormous transition dipole moment, semiconductor nanocrystals are much more absorbing than conventional fluorescent dyes, which makes them ultrasensitive imaging probes. The luminescent properties of nanocrystals result from quantum-size confinement, which occurs when semiconductor particles are smaller than their exciton Bohr radius (1-5 nm diameter). Therefore, their tunable emission wavelength can be tailored into the NIR rather than the visible region of the spectrum where most conventional fluorescent dyes emit. For example, CdSe nanocrystals with ZnS or silica passivation layers are strongly luminescent (up to 80% quantum yield) at room temperature and their emission wavelength can be adjusted from the blue to the red in the visible spectrum by changing the nanocrystal size.

Bio-inspired Nanotechnology

Nature is a main inspirational source for nanotechnology in that it forms from miniature building blocks more complex systems like cells, functional tissues, and living systems. The prime example of nature's nanotechnological capabilities is the harnessing of solar power into chemical energy by the biological photosynthetic machinery in plants. In the same way as evolution has improved the efficiency of biochemical processes over time, bio-inspired nanomaterials has the potential to revolutionize the fields of functional materials, medicine and biotechnology. The ability to precisely manipulate materials on the molecular scale will allow the probing of nanometer-scaled biosystems and ultimately provide the ability to control the fundamental processes in biological systems. In turn, this will enable scientists to manipulate and increase the efficiency of chemical reactions and develop diagnostic tools that can be placed even inside living cells to achieve previously unimaginable accomplishments in the biomedical field.
However, before we can build efficient nanoscale tools and machineries, it is necessary to first understand the processes that occur on a molecular level. For many biological processes the underlying mechanisms remain incompletely understood. For example, in the photodynamic therapy (PDT) of cancer tumors, the mechanisms by which PDT destroys the tumor cells have been previously investigated, but many questions still remain to be addressed. Due to the complexity of the processes in living cells it can be advantageous to first study model systems and apply the obtained insights towards the investigation of living systems. A novel tool that has been successfully applied to study biological systems is Förster Resonance Energy Transfer (FRET). Although molecular FRET has been known for many years, small effort has been so far devoted in studying nanomaterials-based FRET. The unique tunable properties of materials on the nanometer size range will open up new possibilities to exploit the FRET phenomenon in novel ways and bring forth the development of a tool that can aid in understanding the biology of living systems.

Nano-heterostructures:

Core/shell nano-heterostructures are prepared by a one-pot synthesis process. The core/shell nanoparticles show additional tunability in absorption and emission compared to the nanoparticle core. Optimal reaction conditions to form the heterostructured nanoparticles are evaluated and high photoluminescence yields are obtained. Using nanosecond and femtosecond laser spectroscopy, the photoluminescence lifetimes, Stokes shifts, and quantum yields are investigated in the light of quantum confinement, electronic confinement, and interface stress effects.

Laser Spectroscopy:

It is crucial to gain understanding about the rates of the competing relaxation pathways within the nanomaterials. Using femtosecond (fs) and nanosecond (ns) time-resolved laser spectroscopy, measurements will provide a direct and versatile approach for probing the electronic nature of the nanoparticle-based systems. The Burda group has completed setting up a laser laboratory, which is equipped to perform such measurements. The optical properties of nanomaterials are studied on time scales from 100 fs and longer. This allows the monitoring of the electron and hole dynamics in real time. To investigate the surface and bulk properties of the new materials we also employ steady-state and time-resolved FTIR spectroscopy, as well as surface-sensitive photoacoustic IR spectroscopy.

Femtosecond Spectroscopy from the Visible to IR Range

Using a Clark CPA 2001 femtosecond laser, near-Fourier-limited femtosecond laser pulses of 120 fs duration excite the electrons of the nanoparticles from the valence to the conduction band. Subsequently, the electron and hole dynamics is monitored in real time with a 14 fs intrinsic time-resolution (delay line resolution) over a spectral window from 400 nm to 11,000 nm. This allows the monitoring of the electron relaxation on the fast time scales up to 3 nanoseconds. For example, electron cooling and electron-phonon coupling, which are important processes for the design of efficient photovoltaics take place within 100 picoseconds after excitation. Moreover, information whether the charge carriers are efficiently transported and delocalized can be obtained with time-resolved measurements. The laser laboratory is equipped to study the energetics and dynamics of photogenerated excitons, biexcitons, and multiphotonic carrier droplets. The often-occurring Auger processes in nanostructures are also analyzed with this femtosecond spectroscopic set-up. Reflective ultrafast optics is used to avoid dispersion of the probe pulse. An optical parametric amplifier (TOPAS) is in use to generate tunable wavelengths, which enables the investigation of the photo-current and carrier mobility as a function of excitation wavelength.

The ground state absorption from one band level to another can be monitored in the visible range. However, for excited state intraband transitions IR probing is utilized since the transition energies in this spectral range are small (< 0.6 eV, or . > 2000 nm). This involves advanced experiments with visible pumping and mid-IR probing using difference-frequency-converted femtosecond laser pulses. Our IR-probing ability is specifically designed in order to observe the dynamics of intraband transitions (electron and hole cooling). In addition, intra-band gap states, which are caused by dopants or interface-related lattice defects, lead to transitions, which can also escape the visible range and are only observable in the NIR-to-mid-IR range. Therefore, it is important to have this state-of-the-art facility in house for full nanomaterials characterization.

Nanosecond Spectroscopy with High Spectral Resolution

Using a Spectra Physics YAG-MOPO laser system, which has the capability of producing ultra-narrow laser lines, optical hole-burning experiments are conducted. This can be performed dynamically (7 ns laser pulse duration) at room temperature or quasi-statically at very low temperature < 10 K in an optical cryostat. Ultra-narrow laser excitation leads to a narrowing of the spectral response, because only a small homogeneous sub-set of the sample is excited.

With optical hole burning experiments it is possible to resolve the energy levels and the dynamics of electronic states in ensembles of colloidal semiconductor nanocrystals, both in solutions and in matrix. An Andor Intensified Charge Coupled Device (ICCD) camera is in use to record the ns-transient spectra as a function of delay time. The time domain from nanoseconds to milliseconds is of increasing importance as nanoparticles and nanomaterials of increasing quality are produced. For defect-free nanoparticles excited-state lifetimes of several ms are possible.

FTIR Spectroscopy: Steady State and Time-resolved

A Nicolet FTIR spectrometer (Nexus 870) with detection range from 12,000 cm-1 (1000 nm) to 350 cm-1 (20,000 nm) is used for the characterization of interface properties of the nanoparticles. It has been previously demonstrated how the frequencies of adsorbed molecules red-shift when they bind to the nanoparticle surface. Such transition shifts are used diagnostically to determine nanoparticle-matrix interactions. The success of doped nanomaterials and ternary compositions is investigated with photoacoustic IR depth-profiling using the step-scan technique. Our IR set-up also allows to record FTIR spectra with a 10 ns time-resolution.