The overall goal of this procedure is to reliably measure the sharp Raman resonances of one-dimensional systems, as a function of sample temperature. This method can help answer key questions in the field of nanowires, such as the energies of the vibrational and optical excitations, and the nature and quality of samples. The main advantage of this technique is that it allows the direct association of vibrational modes, and electronic transition states of nanowires in samples which contain more than one structure.
Joe Spencer, a grad student in my lab, will be demonstrating the Raman experiments. To begin this procedure, preheat approximately 50 milligrams of single-walled carbon nanotubes to 450 degrees Celsius in dry air. In an argon glove box, grind approximately 20 milligrams of the pre-heated nanotubes with an equal volume of the filling material, for more than 20 minutes using an agate mortar and pestle, applying force to produce an intimate mixture.
While in the glove box, transfer the entire amount of the mixture to a silica quartz ampoule, sealed at one end and open at the other. Seal the ampoule containing the nanotube mixture under moderate vacuum. In a muffle furnace, heat the sealed ampoule at a ramp rate of approximately five degrees Celsius per minute, to approximately 100 degrees Celsius greater than the melting point of the filling material.
After removing the cooled ampoule from the furnace, break it open for further use. After preparing the sample for Raman spectroscopy, set the incident wavelength to the desired value using a tunable laser source, as per the manufacturer's protocol. Rotate the volume brag grating, or VBG, around the vertical axis to reduce the transmission of the laser through the VBG.
Then, fine-tune using the VBG mirror mount. Position the mirror into the brag-reflected beam and retro-reflect the beam back onto the VBG. Adjust the mirror to suppress transmission of the retro-reflected beam through the VBG.
Following this, measure the laser power transmitted through iris one. Then fine-adjust the VBG and retro-reflecting mirror to maximize the transmission of laser power. Adjust the post-filter mirrors to return the laser beam to the predefined path, by repositioning the reflections from the relevant beam splitters onto the two beam observation cameras.
If the laser is not positioned on the same sample point, it can cause greater than 50%variation in signal due to a change in sampling area and how the light is coupled into the spectrometer. Measure the laser photon energy by indirectly scattering into the spectrometer. Adjust the half-wave plate to set the power incident upon the objective to approximately one milliwatt.
Using the imaging optics, check the sample image, and ensure the laser spot is in the desired location with no stigmation. Adjust the sample position so that the laser spot is focused on a clean area of silicon. At this point, set the spectrometer to the zeroth order.
Use the observation camera built into the spectrometer to view the image of the input slit on the first stage of the spectrometer. Set up the spectrometer software as per the manufacturer's protocol, to collect Raman scattering from the silicon Raman peak. Set the power to 10 milliwatts.
Following this, take repeated Raman spectra with one-second exposures to begin focusing. Then, adjust the Z focus of the sample until a well-defined silicon peak at 520 inverse centimeters is observed. Maximize this signal by adjusting the input half wave plate, the input lens, and the Z focus of the sample.
Set the desired temperature to four Kelvin, and allow the system to equilibrate for approximately 40 minutes. Acquire Raman spectra with a charge-coupled device focusing on one-second exposures, as per the manufacturer's protocol. Adjust the Z focus position of the sample using stage controllers to maximize the reflected power at the power meter.
Finally, acquire a Raman spectrum using suitable exposure time to get a sufficient signal. Several peaks are observed in the Raman spectra of the extreme nanowires, which can be attributed to the vibrational excitations, including both one phonon and multi-phonon Raman peaks. A key indicator that specific Raman features are associated with nanowires, rather than nanoparticles or lumps of the parent material, is a characteristic polarization dependence like that shown here.
The Raman scattering from an ensemble of randomly-oriented one-dimensional systems is preferentially polarized in the same direction as the exciting laser light, with a contrast ratio of three to one, and shows the characteristic figure eight shape. Representative measurements of the excitation intensity dependence of the Raman scattering intensity of Mercury telluride extreme nanowires, are shown here. The Raman intensity initially increases linearly, before starting to show non linear behavior with a tendency for the signal to saturate.
As the temperature increases, the spectra width broadens, and the center shift of the mode softens. The drop-off in intensity as a function of temperature is predominantly due to a decrease in the coherent lifetime of the optical states responsible for the resonance with increasing temperature, and is clear evidence that Raman scattering can provide information far beyond that possible with absorption measurements. While attempting this procedure, it's important to remember to monitor the repeatability of the process.
Following this procedure systematically should allow a user to obtain repeat measurements of the same wavelength to within 10%of the Raman scattering intensity.
