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Materials Science

Materials science is an interdisciplinary field that investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. Materials science aims at creating or transforming existing materials as well as enhancing materials to give better performance for particular applications.


Time-Resolved Photoluminescence (TRPL)

From picoseconds to milliseconds

The fluorescence (or more generally the photoluminescence) lifetime is an intrinsic characteristic of a luminescent species that can provide insight into the species excited state dynamics. Time-Resolved Photoluminescence (TRPL) is the tool of choice for studying fast electronic deactivation processes that result in the emission of photons, a process called fluorescence. The lifetime of a molecule in its lowest excited singlet state usually ranges from a few picoseconds up to nanoseconds. This fluorescence lifetime can be influenced by the molecular environment (e.g., solvent, presence of quenchers (O2), or temperature) as well as interactions with other molecules. Processes like Förster Resonance Energy Transfer (FRET), quenching, solvation dynamics, or molecular rotation also have an effect on the decay kinetics. Lifetime changes can therefore provide information about the local chemical environment or insights into reaction mechanisms.

Some species such as metal-organic complexes, inorganic crystal structures, semiconductors and new types of hybrid materials have emission lifetimes ranging from nano- to micro- or even up to milliseconds. In this case the luminescent species relaxes from its lowest excited triplet state by emitting a photon in a process called phosphorescence.

Time-Correlated Single Photon Counting (TCSPC) is a popular method for carrying out TRPL measurements. TCSPC works by measuring the time between sample excitation by a laser pulse and the arrival of the emitted photon at the detector. TCSPC requires a defined “start”, provided by the electronics steering the laser pulse or a photo diode, and a defined “stop” signal, realized by detection with single-photon sensitive detectors. The measurement of this time delay is repeated many times to account for the statistical nature of the fluorophores emission. The detected events are then sorted into a histogram according to their arrival time which allows reconstruction of the photoluminescence decay.

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Time-Resolved Photoluminescence (TRPL)

TRPL Imaging

Direct observation of charge carrier dynamics

The general methodology of time-resolved photoluminescence can be expanded by lifetime imaging of the charge carrier dynamics. This can be exploited for, e.g., determining the effect of carrier diffusion and its influence on the total lifetime measured in conjunction with intensity dependent photoluminescence lifetimes measurements. It brings an exceptional component to semiconductor analysis with respect to material and architectural substructures, spatial inhomogenities and process dependent morphology. Using TRPL imaging, charge carrier diffusion processes and the effect of localized inhomogeneities and defect sites can be identified. With this multi-dimensional approach, a versatile and powerful methodology for the analysis of semiconductor materials can be achieved.

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TRPL Imaging

Lanthanide Upconversion

Upconversion photoluminescence of lanthanide complexes

Lanthanide-doped upconversion materials have great potential for applications such as fluorescence labels for in vitro bioimaging, as lighting sources in optical devices or as up-shifting layers in solar cells. These materials absorb light in the near infrared (NIR) spectral region (typically at ca. 980 nm) and emit light in the visible range. The structure of the luminescence spectrum strongly depends on the composition of the upconversion material as well as on the excitation intensity. The FluoTime 300 spectrometer can achieve very high spectral resolution along with outstanding stray light rejection when equipped with double monochromators in both excitation and emission pathways. This configuration is therefore well suited to measure the rather weak upconversion luminescence from these highly scattering samples. The upconversion luminescence kinetics of lanthanide-doped materials can range from nanoseconds to milliseconds. Thus the FluoTime 300 spectrometer is ideally suited to study these materials as it can cover time spans from a few picoseconds to several seconds by using either Time-Correlated Single Photon Counting (TCSPC) or Multi-Channel Scaling (MCS).

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Lanthanide Upconversion

Quantum Dots and Nanoparticles

Investigation of Nano Materials

Quantum dots (QD) are semiconductor particles with sizes of a few nm. QD emit light of a specific wavelength when a current is applied or exposed to light. The emission wavelength can be tuned by changing either the size, shape, material, or by doping the QDs. Smaller QDs (2–3 nm) emit light at short wavelengths (blue-green spectral region), while larger QDs (5–6 nm) will emit light in the longer wavelengths (orange, red, or IR). Furthermore, it has been shown that their fluorescence lifetime is also tied to particle size. In larger dots, the lifetime is longer due to more closely spaced energy levels in which the electron-hole pair can be trapped.

Nanoparticles (NPs) are also very small structures but larger than QDs, usually ranging from 8 to 100 nanometers. Because of this, NPs exhibit behaviors between those bulk materials and atoms or molecules. NPs often possess unexpected optical properties as their size allows for quantum confinement effects. Additionally, the interfacial layers surrounding NPs play an important role in all of their physical properties. These layers typically consists of ions, inorganic material, or organic molecules.

By controlling their size, shape as well as composition, the absorption properties of NPs can be fine-tuned to fit the needs of photovoltaic or solar thermal applications. QDs are also of great interest for display or lighting applications where their stability and tunable emission properties are very desirable.

Both time-resolved as well as steady-state luminescence spectroscopy are excellent tools for investigating the excited state characteristics and dynamics of both NPs and QDs.

Typical application areas for such materials are:

  • LEDs
  • Solar cell
  • Diode lasers and second-harmonic generation
  • Displays
  • Photodetectors
  • Photocatalysts
  • Transistors
  • Quantum computing
  • Medical applications as imaging markers, tumors detection or photodynamic therapy
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Quantum Dots and Nanoparticles

Semiconductor Materials

Time-resolved spectroscopy of semiconductor materials

Charge carrier dynamics in semiconductors are determined by the architecture and function of the respective device. Thus directly reflecting the nature and quality of wafer materials. Understanding these critical parameters in semiconductor photophysics is of the utmost importance. A precise and efficient measurement of the diffusion lengths of photo-excited electrons and holes is essential for characterizing these systems. Time-resolved photoluminescence quenching experiments are a valuable tool for determining such diffusion lengths.

For specific classes of semiconductors, the characteristic charge carrier lifetime depends strongly on the nature and dimensions of the materials and interfaces involved. Furthermore, surface effects, passivation, energy transfer efficiency of sensitizers as well as the presence of dopants, impurities, and defect sites can also introduce significant variations in the observed lifetimes.

As photoluminescence behavior of a semiconductor is a direct monitor for its charge carrier dynamics, time-resolved photo-luminescence (TRPL) via time-correlated single photon counting (TCSPC) is well suited for analyzing phenomena that influence its dynamics. As a result, the phenomena occurring within a particular system can be characterized directly down to the sub-nanosecond time scale.

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Semiconductor Materials

Singlet Oxygen Luminescence

Steady-state and time-resolved singlet oxygen luminescence

Singlet oxygen is a highly reactive species that can be generated by illuminating organic dyes (called photosensitizers) under aerobic conditions. This reactive species plays an important role in many photoinduced oxidative processes in both biology and chemistry, and is responsible, for example, for photochemical degradation of various materials or for destroying cancer cells during photodynamic therapy.

An excellent way to detect the presence of singlet oxygen are steady-state or time-resolved measurements of its characteristic phosphorescence at around 1270 nm. However, such measurements can be quite challenging due to the rather weak phosphorescence intensities (i.e. low emission quantum yield of singlet oxygen). The phosphorescence lifetime of singlet oxygen is solvent dependent and can therefore be used to gain information about the environment of the emitting oxygen molecules.

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Singlet Oxygen Luminescence

Laser Seeding

Laser output injected into an amplifier or another laser

A seed laser is a laser whose output is injected into some amplifier or another laser. Seed lasers are typically combined with an amplifier in a master oscillator power amplifier configuration used for generating an output with high power. The seed laser approach is often superior to a direct high power laser as very often certain features of low power seed lasers such as short pulses, adjustable repetition rates or narrow spectral line widths are easier to obtain.

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Laser Seeding