Name: resonance raman spectroscopy of extreme nanowires and other 1d systems
Uploaded: 2016-04-28T14:00:00
Duration: 7 min 44 s
Description: Watch this Scientific Journal Video about Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems at JoVE.com

The authors acknowledge financial support from the Engineering and Physical Sciences Research Council, UK under the Program Grant 'Supercritical Fluid Electrodeposition' (EP/J016276/1). J.S. and R.J.K. are indebted to the Warwick Centre for Analytical Science (EPSRC funded Grant EP/F034210/1). Additionally, we are indebted to Drs. Zheng Liu and Kazu Suenaga who provided the top right part of Panel d of Figure 1, which originally appeared in Microsc. Semicond. Mater. 2008, 120, 213-216 (used with permission).

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The authors have nothing to disclose.

Whilst a huge amount of research has been done on nanowires the fundamental limit of the smallest diameter nanowires possible, extreme nanowires, has hardly been explored. It has already been shown that the properties of these nanowires do not form a continuum with even slightly larger diameter nanowires; e.g. they can exhibit entirely new crystalline forms of their parent materials. Considering the large number of possible parent materials and that each parent can produce many more than one extreme nanowire the range of possible nanowires physics is huge.
The fact that extreme nanowire research is still in its early stages is not because the methods of producing them are not well established. The melt infiltration process set out in this paper is reliable and has been used by many groups and other approaches such as sublimation filling are available if melt infiltration is not optimal for any particular filling. In part the field is held back by the lack of a relatively simple and widely applicable method for non-destructively characterizing extreme nanowires. If the field of carbon nanotubes is any guide, Raman spectroscopy has a good chance of being the method of choice for solving this problem. The key to obtaining useful Raman spectra on extreme nanowires is to recognize that in common with all other 1D systems resonant enhancement of the Raman scattering is a necessary condition for observing any scattering. Once the full resonance behavior of a particular sample type has been determined using the methods set out in this protocol it is possible to use a fixed resonant excitation energy for most applications of Raman to characterizing the sample that will speed up the measurements and reduce the cost of the Raman system required.
As shown in the results presented in this paper the critical problem in obtaining high quality Resonance Raman results on extreme nanowires is the need to be able to reproducibly realign the beam of a tunable laser over several days with high precision. This requires particular modifications to the experimental system and attention to the most important details of the experiment; correct focusing of the optical system, precise alignment of the laser beam onto the microscope objective and the ability to correct precisely for any lateral movement of the sample. The techniques developed to achieve this form the basis of this paper. Others have developed techniques and systems for improving the reproducibility of resonant Raman experiments including pioneers such as M. Cardona who applied the technique to a wide range of bulk and quantum well systems. Our technique also builds upon the work of the pioneers of Raman in carbon nanotubes including M. Dresselhaus21. However the protocol presented here is particularly suitable for Resonance Raman experiments on extreme nanowires.
A key part of the success of the protocol was the development of the experimental system shown in Figure 10. The figure demonstrates a plan view of the optical setup employed for the Raman experiments detailed in the protocol. Laser light is focused through a 50X objective (labeled OB) upon the sample, sealed in the cryostat as per the protocol. This cryostat is mounted on an XYZ stage to allow 3 dimensional movement of the sample for purposes of repositioning and focusing. Laser light is generated through A and B (being a pump source and Ti:sapphire respectively), exact details of the laser being noted in the materials document provided. When using the commercial laser line filter (component C) laser light is directed through the center of iris 1 and 2 and collimated using lens 1 and 2 (L1 and L2). The light passes through a half-wave plate and polarizer (HWP1 and Pol1) to control plane of polarization and laser power incident upon PM2, as detailed in the protocol. Laser light is passed through the tunable filter, C, and using mirrors M1 and M2, steered onto the correct optical path such that it is normal to the back face of the objective (OB) and centered on the cameras C1 and C2. The ND filter is used to position the back-reflected beam from the objective onto power meter, PM1, to allow the focusing procedure (step 9.9) to be performed. Back scattered light from the sample is collected and passed through lens 3 (L3) and Slit 1 into the spectrometer. Adjusting the slit width and position of the lens is important to maximize the Raman signal, as detailed in protocol section 8. If the laser wavelength is out of the laser line filters operational range, the Volume Bragg setup needs to be employed as per section 8.2.1-8.2.3. It is important that the optical set up is changed in accordance with the black dashed line as per Figure 10, and the mirror M3 is removed from the path. Finally, if undertaking polarization dependent experiments, it is important to control the polarization and maintain the polarization entering the spectrometer, this is explained in section 12 of the protocol and components to be added to the setup are highlighted by a purple dashed line in Figure 10. The blue dashed line in Figure 10 indicated components that are added to allow live imaging of the sample as indicated by section 14 of the protocol.
As with all experimental methods Resonant Raman scattering has its limitations. In particular, the available tunable laser sources and detectors mean that it is much easier to perform in the spectral range 350-1,000 nm although extension further into the infrared and UV are possible. The experimental system required to undertake Raman scattering with tunable sources is not cheap with a reasonable estimate being &#163;200-300k at the time of publication. In addition the complexity of the systems required means that they require some familiarity with optical spectroscopy to operate successfully. However Raman scattering provides a combination of information that is hard to obtain from other techniques. Remarkably it is possible to obtain Raman scattering, and thus vibrational energies, from individual single walled carbon nanotubes that cannot yet be achieved by any other technique.
Now that the resonances of nanowires are starting to be determined this opens up a range of possible extensions of Raman scattering. In our opinion the extension to electrochemically gated extreme nanowires20 at temperatures down to 4 K36, allowing measurements on nanowires over a wide range of charge densities will be key to understanding these materials. Finally using Raman scattering to understanding structural and melting transitions of extreme nanowires may help to optimize the quality of the samples that can be produced even further.

Raman spectroscopy and Resonance Raman spectroscopy are well-established techniques that are widely exploited scientifically and technologically. Whilst first reported by Raman himself in 19281 the key to wide spread use of Raman spectroscopy was the development of lasers, tunable lasers in the case of Resonance Raman, to provide high intensity, narrow bandwidth excitation sources. This paper sets out why Resonance Raman scattering is a particularly important method for investigating the fundamental physics and characterizing samples of 1D systems in general and extreme nanowires, e.g. nanowires with diameters of ~1-5 atoms. It also discusses difficulties particular to Raman spectroscopy of such nanowires and a protocol that allows these to be overcome and thereby achieve high repeatability measurements of the laser energy dependence of the Raman scattering efficiency in these systems.
There is a wide range of extended, crystalline 1D quantum systems, also known as nanowires, available for study and application. These include vapor-liquid-solid grown semiconductor nanowires2, lithographically defined nanowires3, anodic alumina and track etch membrane template nanowires4 and others. A key reason for the interest in these systems is that they combine large quantum confinement effects with the ability for electrons and other excitations to move freely along the structure. In some respects nanowires are quite different from their parent material, e.g. reduced electromagnetic screening due to free charges5, and in some cases reduced electron scattering leading to ballistic transport6. However, in many respects the nanowires are still bulk like, e.g. the local bonding and crystal structure, and nearly always the fundamental quality of the electronic wave functions at the atomic scale are only weakly modified compared with bulk so that the envelope approximation7 is valid. However as the dimensions of the confined directions are reduced to a few atoms, nanowires with entirely new bonding can occur forming never previously seen allotropes8-10. These nanowires are extreme in two senses; they are at the extreme limit of the possible reduction in cross-section11-13 and they have extreme properties10,13,14.
Before undertaking Resonance Raman spectroscopy, it is necessary to produce the extreme nanowire samples. The methodology set out in this paper for generating these nanowires is the melt infiltration of materials into single walled carbon nanotubes. Melt infiltration is one of two-high yield filling protocols used for obtaining continuously filled single-walled carbon nanotubes (SWNT), the other being sublimation, which is popular for the introduction of some molecules (i.e. fullerenes) and some binary salts, most recently CsI13. While the latter method produces near quantitative filling, it is limited in that the material to be introduced must readily sublime which greatly constrains the number and type of fillings that may be introduced into SWNT. The melt infiltration filling protocol can, with care, be used to produce near quantitative filling15 and has fewer constraints than that of the sublimation protocol. These are that the material must have a surface tension lower than 100-200 mN m-1 and a melt temperature below about 1,300 K to avoid damaging the host SWNTs.16
Transmission electron microscopy (TEM) is the best method to characterize the quality of the filling of the carbon nanotubes and identify the crystalline structure or structures of the extreme nanowires produced. Solving structures of SWNT-embedded crystal fragments from HRTEM images involves trial-and-error comparisons between image simulations from trial crystal fragment models and the experimentally obtained image contrast. This paper describes a protocol for confirming the microstructure of the extreme nanowire motifs in SWNT samples by HRTEM image simulation as a prelude to their spectroscopic characterization.
Resonance Raman spectroscopy17 is an ideal tool both for understanding the fundamental physics of extreme nanowires and, once the resonance energies have been determined, for characterizing the type and quality of samples of nanowires. Fundamentally, Resonance Raman allows the direct determination of both optical and vibrational excitation energies17. With additional modeling of the photon energy dependence of the resonance it is possible to quantify the electron-phonon interaction17. Once resonant energies have been determined for particular extreme nanowires, the Raman spectrum of the nanowires can be used to track strain18 and structural phase changes19 due to temperature, hydrostatic pressure, or bending of the wire. Whilst it still to be proven, it is likely that in some magnetic extreme nanowires spin excitations will lead to Raman scattering allowing them to be probed. Extension of Raman scattering to samples held in a spectroelectrochemical cell can be used to probe charge transfer between extreme nanowires and host nanotubes20. As a characterization tool Raman spectroscopy provides a method for non-contact, non-destructive determination of nanowire type and quality21. It can be used as a tool for characterizing samples after production and/or purification and even when the nanowires have been included in devices such as transistors or composites which are at least partially transparent at the necessary photon energies.
There is no one technique that can provide a direct alternative for Resonance Raman scattering (RRS); however there are a range of other techniques that overlap some aspects of the capabilities this method. In terms of determining the optical transition energies of extreme nanowires UV-VIS-NIR absorption measurements22 offer a much simpler technique. However in samples with an ensemble of different structures absorption spectroscopy cannot separate the different optical features into sets associated with particular structures. Resonance Raman scattering can achieve this due to the association of optical and vibrational spectra. A combination of the two techniques in which a UV-VIS-NIR absorption measurement highlights target energies of Resonance Raman can speed up the overall process considerably. Photoluminescence excitation spectroscopy (PLE)23 does offer the ability to associate different optical transitions in a single sample; however it only works for some, particularly non-metallic nanowires, and it is only slightly less complicated to perform than RRS and in general requires mono-dispersed samples protected from the environment to be completely successful. Unlike PLE, Resonance Raman spectroscopy works equally well with bundled and mono-dispersed samples and therefore requires little sample preparation. Whilst as yet little used, Rayleigh scattering spectroscopy on individual nanowires24 followed by Transmission Electron Microscope (TEM) analysis of the structure of the nanowire can identify all of the optical excitation energies of the wire in the spectral range investigated and identify a particular nanowire structure. However, this technique does not provide the vibrational energy information possible with RRS; it is very challenging to perform and is never going to be suitable as a general characterization tool. In terms of the vibrational energy information the only currently viable alternative is IR spectroscopy25 however this is likely, due to selection rules, to probe a different set of vibrational energies and thus be complementary rather than competitive. In addition IR spectroscopy will suffer from the same issues with ensemble samples as UV-VIS-NIR absorption measurements.
As already discussed Raman spectroscopy has been applied to a wide range of problems within science. In molecular systems it is used to complement IR spectroscopy for determining vibrational spectra and also as a fingerprinting technique for analyzing the composition of materials. It has been widely exploited in crystalline systems, e.g., the Light Scattering in Solids series of books includes nine volumes. In the case of 3D and 2D systems, resonant excitation is used less for enhancing the overall scattering intensity and more for enhancing the contribution of specific optical transitions within the Raman process leading to the breakdown of the standard selection rules and the ability to quantify the interaction of the excitations observed in the Raman spectrum with specific electronic states. More recently Raman spectroscopy has been central to the study of carbon nanotubes, particularly single walled carbon nanotubes. The carbon nanotube research21 has highlighted the fact that for 1D systems resonant excitation is not optional, as it is for most applications of Raman for 3D and 3D systems, but is strictly necessary. This is because non-resonant Raman scattering is too weak to be observed and it is only when the excitation is resonant with the strong van Hove singularities in the optical density of states, that are a feature of 1D systems in particular, that any Raman spectrum can be observed. Thus in the case of extreme nanowires the use of Raman spectroscopy requires a full Resonance Raman measurement to find the resonances of all of the nanowires in a sample before Raman spectroscopy can be applied to studying these materials.

<table>
	<tbody>
		<tr>
			<td>Carbon Nanotubes</td>
			<td>Nanointegris</td>
			<td>NI96</td>
			<td></td>
		</tr>
		<tr>
			<td>Carbon Nanotubes</td>
			<td>Private</td>
			<td></td>
			<td>Synthesis described in Eurasian Chem.-Technol. J. 2005, 5, 7-18.</td>
		</tr>
		<tr>
			<td>Mercury Telluride</td>
			<td>VMR</td>
			<td></td>
			<td>99.999% metals basis</td>
		</tr>
		<tr>
			<td>Silica Quartz Tubing</td>
			<td>H. Baumbach &#38; Co.&#160;</td>
			<td></td>
			<td>Various diameters and lengths used - typically 1 cm OD, 0.8 cm ID and 8cm long.&#160;</td>
		</tr>
		<tr>
			<td>Tube furnace</td>
			<td>Carbolite</td>
			<td>&#160;MTF-12/38/250</td>
			<td></td>
		</tr>
		<tr>
			<td>JEOL ARM 200F&#160;</td>
			<td>JEOL&#160;</td>
			<td></td>
			<td>200 kV High Resolution TEM Operated at 80 kV and equipped with&#160;</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>CEOS hardware spherical aberation (Cs) imaging corrector. Cs corrected&#160;</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>to 0.001 mm.</td>
		</tr>
		<tr>
			<td>SC1000 ORIUS camera</td>
			<td>Gatan</td>
			<td></td>
			<td>Size of CCD 4008 x 2672</td>
		</tr>
		<tr>
			<td>Digital Micrograph Suite 2.31</td>
			<td>Gatan</td>
			<td></td>
			<td>64 bit version</td>
		</tr>
		<tr>
			<td>XMax X-ray Microanalysis&#160;</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td>This detector uses the silicon drift detection (SDD) principle. 1 nm diameter electron probe.</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Crystalmaker Ver 8.7</td>
			<td>Crystalmaker</td>
			<td></td>
			<td>Used for assembling crystal fragments for image simulations</td>
		</tr>
		<tr>
			<td>Nanotube Modeler</td>
			<td colspan="2">JCrystalSoft &#169;2015-2015</td>
			<td>Used for generating Nanotube models</td>
		</tr>
		<tr>
			<td>SimulaTEM</td>
			<td>Private</td>
			<td></td>
			<td>Ultramicroscopy, 2010, 110, 95-104.</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Verdi V8 Pump</td>
			<td>Coherent</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mira 900 Ti:Sapphire</td>
			<td>Coherent</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Volume Bragg Grating</td>
			<td>Optigrate</td>
			<td></td>
			<td>Specfication between 680-720nm</td>
		</tr>
		<tr>
			<td>Photonetc TLS 850 LLTF&#160;</td>
			<td>Photonetc</td>
			<td></td>
			<td>Tunable between 700-1000nm</td>
		</tr>
		<tr>
			<td>LMPLAN IR50x MircoscopeObjective</td>
			<td>Olympus</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cryostat</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Triple Raman Spectrometers</td>
			<td>Princeton Instruments</td>
			<td></td>
			<td>triple 600nm using gratings 900, 900, 1800 lines/mm</td>
		</tr>
		<tr>
			<td>CCD</td>
			<td>Princeton Instruments</td>
			<td></td>
			<td>deep depleted, UV enchanced liquid N2 Cooled Silicon CCD</td>
		</tr>
	</tbody>
</table>

resonance raman spectroscopy of extreme nanowires and other 1d systems

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.

Watch this Scientific Journal Video about Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems at JoVE.com

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

The paper describes a method for producing extreme nanowires by melt infiltration into carbon nanotubes and how 1D systems may be characterized and investigated using Resonance Raman Spectroscopy to determine vibrational and optical excitation energies.

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the ...

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