Applied Physics Research Lab

Goals: The major goals of the proposed project are (i) to develop advanced research based experiments on nanomaterials and optical as well as electron microscopic characterization techniques, (ii) to develop a manual describing the procedures, and (iii) to prepare students for careers in nanomaterials/photonics industry.

Objectives: The main objective is to promote the professional development of undergraduate students in optical nanomaterials science and technology. Several advanced experimental projects will be designed with emphasis on “learning multidisciplinary skills by doing” especially to train undergraduate students for successful careers in the nation's scientific and technological infrastructure in nanomaterials. Such hands-on experiences will allow them to: i) enhance their research and academic training in materials science, condensed matter physics, and nondestructive characterization techniques (optical and nonoptical); ii) develop research, methodological, theoretical, analytical, and critical thinking skills; iii) develop scientific writing and presentation skills.

Photoacoustic (PA) technique has widely been used to investigate samples with relatively low fluorescence quantum efficiency. The major advantage of this technique is that “as received” samples in any state can be investigated with greater sensitivity. The principles and applications in various fields are well documented in the literature [for example, A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, John Wiley & Sons, New York, 1980.). Briefly, the sample is excited to higher electronic excited states by intensity modulated light radiation. Nonradiative decay to the ground state leads to heat generation modulated at the same frequency, which in turn leads to pressure oscillations in the gas that surrounds the sample. The pressure oscillations are sensed by a microphone, processed and plotted as a function of wavelength. The strength of the acoustic signal is proportional to the amount of light absorbed by the sample and then there is a close correspondence between PA spectrum and the conventional optical absorption spectrum.

(1) 125 W High intensity water cooled ceramic IR source (range 9,600-50 cm^-1), (2) PERMATRAC 2™ A 60 degree Michelson air bearing interferometer, a design found only on the highest quality research grade spectrometers. The PERMATRAC 2™ interferometer is dynamically aligned by patented Piezo-stack technologies. The PERMATRAC 2™ interferometer in combination with the 125 W water-cooled source deliver an unprecedented 160 mW of IR power to the sample position.

(3) 0.10 cm^-1 resolution at 4,000 cm^-1.

(4) Step scan capability - provides scan speeds from 800 Hz to 1 step every 250 seconds. In the amplitude modulation mode the FTS 7000 can be used with source (e.g. chopper) or sample modulation with an external lock-in amplifier. Phase modulation is also available, at frequencies up to 1000 Hz. Phase modulation measurements are performed in conjunction with DSP. Step scan also includes operation in time resolved spectroscopy mode "TRS", with 5 microsecond time resolution and event trigger.

(5) Digital Signal Processing (DSP3) for modulation experiments. Provides three modes of operation; DSP1, DSP2 and DSP3. DSP1 demodulates the sample response in a step-scan photoacoustic measurement with phase modulation. It is a replacement for the Varian demodulator board or a lock-in amplifier for PAS applications, providing the sample in-phase and in-quadrature response to the phase modulation of the spectrometer. In addition, it provides for the simultaneous demodulation at odd harmonics of the phase modulation frequency, up to the ninth harmonic, providing the sample response to five effective phase modulation frequencies in one measurement. DSP2 demodulates the sample response to simultaneous phase modulation and a lower frequency sample modulation. DSP2 replaces the function of two lock-in amplifiers working in series, providing the in-phase sample response to phase modulation, and the in-phase and quadrature sample response to sample modulation. DSP3 demodulates the signal generated by a photo-elastic modulator (PEM) running at 37 KHz, for either linear dichroism or circular dichroism measurements. DSP3 replaces the function of from one to five lock-in amplifiers, dependent on the experiment. Typical applications are PEM-IRRAS masurements, dynamic infrared linear dichroism (DIRLD) and vibrational circular dichroism (VCD). Includes an electronic bandpass filter and external detector cable. DSP3 is designed for use with a 37 KHz PEM. PEMs at other frequencies are not supported.

Cary Eclipse Fluorescence Spectrophotometer

The Cary Eclipse is a computer-controlled, ratioing fluorescence spectrophotometer with dual monochromators and measurement modes for fluorescence, phosphorescence, chemiluminescence, and bioluminescence. The system uses Cary's unique pulsed lamp technology, like the other Cary spectrophotometers, which gives the Eclipse room light immunity in fluorescence mode. This revolutionary technology allows for easy reagent addition or the use of large, bulky sample handling accessories since the sample compartment can be open during sample measurements. Moreover, the lamp is on only during sample measurement, which preserves the lamp life (guaranteed for 2 yrs of operation) and minimizes photobleaching effects. The short lamp pulse eliminates the release of ozone into the laboratory and the completely enclosed lamp module provides superior efficiency and safety during lamp replacement. The narrow 2 microsecond Xenon lamp pulse (fwhm) and peak power equivalent to 75 kW offer enhanced sensitivity, while still minimizing photobleaching effects. Increased energy throughput results from all reflective, quartz-over coated optics and the use of Schwarzchild collection optics to precisely image and focus the lamp. The excitation and emission monochromators are 0.125 m Czerny-Turner design with an f3.6 focal length and have a limiting resolution of < 1.5 nm. Each monochromator has fixed selectable slits with spectral band passes of 1.5, 2.5, 5, 10, and 20 nm plus 10 nm Round and include as standard, computer-controlled stray light/scatter rejection filters. Specifications for the monochromators include:

The horizontal beam and slit image reduces the amount of sample required for analysis requiring as little as 0.50 mL in a standard 3.0 mL cell. Guaranteed 750:1 RMS using 350 nm excitation, 10 nm slits, 1 s signal averaging 500:1 RMS using 500 nm excitation, 10 nm slits, 1 s signal averaging Sample and reference signals are detected by high performance R928 photomultiplier tubes (PMTs) that are standard. The PMTs can operate with a gain of Low, Medium, or High, with manual adjustment from 400-1000 V in increments of 1 V. The non-measurement, phase stepping wavelength drive eliminates scan speed errors by

synchronizing the lamp pulse and scan motors so that no measurements are taken while the monochromators are moving. The time drive mode also synchronizes the lamp pulse to data acquisition. Allowable data intervals include: 0.15-30 nm; 9.3711-140.0566 cm^-1; 1.5-300 A, 0.0012-0.0174 eV. Data collection rates in points per minute per cell are 4800 points/1 cell; 6 points/4 cells; and 55 points/4 cells with minimum SAT and Dwell Time.

Signal averaging varies with the selected collect mode as follows:

Fluorescence (0.0125 to 999 s), Phosphorescence (0.001 ms to 10 s Gate time), Bio-Chemi-Luminescence (0.040 ms to 10 s Gate time).