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Figure 8.3. GaAs excitonic absorption spectra near the bandgap for several temperatures. The experimental points for the spectra are given, and the dark lines represent theoretical fits to the data. (From M. D. Sturge, Phys. Rev. 127, 768 <1962); see also P. Y. Yu and M. Cardona, Fundamentals of Semiconductors, 3rd ed., Springer, Berlin, 2001, p. 287.]

the free-electron mass mo, as is clear from the data listed in Table B.8. As a result, the effective mass m* = m^n\J{me + mh) of the exciton is significantly less that the mass to0 of a free electron. The material itself has a dielectric constant e listed in Table B.l 1, which is appreciably larger than the value «o in free space, and the result is a system of energy levels that is related to the ground-state hydrogen atom energy 13.6eV through Eq. (2.18):

£=1316ro7ro2eV

where the quantum number n takes on the values n = 1,2,3,4,..., with the value n = 1 for the lowest energy or ground state. It is clear from Eq. (8.2) that both the mass ratio m*/m0 and the dielectric constant ratio s/«o have the effect of decreasing the exciton energy considerably below that of a hydrogen atom, as shown in Fig. 2.20. In bulk semiconductors the absorption spectra from excitons are generally too weak to be observed at room temperature, but can be seen at low temperature. Extensive ionization of excitons at room temperature weakens their absorption. The resulting temperature dependence is illustrated by the series of spectra in Fig. 8.3, which display excitonic absorption near die band edge that becomes more prominent as the temperature is lowered.

Thus far we have discussed the optical absorption of the bulk semiconductor GaAs, and spectroscopic studies of its III-V sister compounds have shown that they exhibit tiie same general type of optical absorption. When nanoparticles are studied by optical spectroscopy, it is found that there is a shift toward higher energies as the size of the particle is decreased; this so-called blue shift is accompanied by an enhancement of the intensity, and the exciton absorption becomes more pronounced. The optical spectra displayed in Fig. 4.20 for CdSe illustrate this trend for die particle sizes 4 and 2 nm. Thus lowering the temperature influences the spectra very much in the same way as decreasing the particle size affects them.

8.2.2. Infrared Surface Spectroscopy

The general principles of infrared (IR) spectroscopy, including Fourier transform infrared spectroscopy (FTIR), were explained in Section 3.4.1. These spectroscopic techniques measure the absorption of radiation by high frequency (i.e., optical branch) phonon vibrations, and they are also sensitive to the presence of particular chemical groups such as hydroxyl (—OH), methyl (-CH3), imido (—NH), and amido (-NH2). Each of these groups absorbs infrared radiation at a characteristic frequency, and the actual frequency of absorption varies somewhat with the environment. We discuss some results based on work of Baraton (2000).

As an example, Fig. 8.4 shows the FTIR spectrum of titania (Ti02), which exhibits IR absorption lines from the groups OH, CO, and C02. Titania is an important catalyst, and IR studies help elucidate catalytic mechanisms of processes that take place on its surface. This material has the anatase crystal structure at room temperature, and can be prepared with high surface areas for use in catalysis. It is a common practice to activate surfaces of catalysts by cleaning and exposure to particular gases in oxidizing or reducing atmospheres at high temperatures to prepare sites where catalytic reactions can take place. The spectrum of Fig. 8.4 was obtained after adsorbing carbon monoxide (CO) at 500°C on an activated titania nanopowder surface, and subtracting the spectrum of the initial activated surface before the adsorption. The strong carbon dioxide (C02) infrared absorption lines in the spectrum show that the adsorbed carbon monoxide had been oxidized to carbon

Figure 8.4. Fourier transform infrared (FTIR) spectrum of activated titania nanopowder with carbon monoxide (CO) adsorbed on the surface. The spectrum of the initial activated titania has been subtracted. The negative (downward) adsorption in the OH region indicates the replacement of hydroxyl groups by C02 on the surface. [From M.-I. Baraton and L. Mertiari, Nanostruct. Mater. 10, 699 (1998).]

Figure 8.4. Fourier transform infrared (FTIR) spectrum of activated titania nanopowder with carbon monoxide (CO) adsorbed on the surface. The spectrum of the initial activated titania has been subtracted. The negative (downward) adsorption in the OH region indicates the replacement of hydroxyl groups by C02 on the surface. [From M.-I. Baraton and L. Mertiari, Nanostruct. Mater. 10, 699 (1998).]

dioxide on the surface. Note that the OH absorption signal is in the negative (downward) direction. This means that the initial activated titan ia surface, whose spectrum had been subtracted, had many more OH groups on it than the same surface after CO adsorption. Apparently OH groups originally present on the surface have been replaced by C02 groups. Also the spectrum exhibits structure in the range from 2100 to 2400 cm-1 due to the vibrational-rotational modes of the CO and C02. In addition, die gradually increasing absorption for decreasing wavenumber shown at the right side of the figure corresponds to a broad spectral band that arises from electron transfer between the valence and conduction bands of the n-type titania semiconductor.

To learn more about an infrared spectrum, the technique of isotopic substitution can be employed. We know from elementary physics that die frequency of a simple harmonic oscillator co of mass m and spring constant C is proportional to (C/m)1/2, which means that die frequency co, and the energy E given by E= fim, both decrease with an increase in the mass m. As a result, isotopic substitution, which involves nuclei of different masses, changes the IR absorption frequencies of chemical groups. Thus the replacements of ordinary hydrogen *H by the heavier isotope deuterium 2D (0.015% abundant), ordinary carbon 12C by 13C (1.11% abundant), ordinary 14N by I5N (0.37% abundant), or ordinary 160 by 170 (0.047% abundant) all increase die mass, and hence decrease the infrared absorption frequency. The decrease is especially pronounced when deuterium is substituted for ordinary hydrogen since the mass ratio mD/mH = 2, so the absorption frequency is expected to decrease by die factor Si 1.414. Hie FTIR spectrum of boron nitride (BN) nanopowder after deuteration (H/D exchange), presented in Fig. 8.5, exhibits this sfl shift The figure shows the initial spectrum (tracing a) of the BN nanopowder aft» activation at 875 K, (tracing b) of the nanopowder after subsequent deuteration, and (tracing c) after subtraction of the two spectra. It is clear that the deuteration converted the initial B—OH, B—NH2, and B2—NH groups on the surface to B—OD, B—ND2, and B2—ND, respectively, and that in each case the shift in wavenumber (i.e., frequency) is close to the expected V5. The overtone bands that vanish in die subtraction of the spectra are due to harmonics of the fundamental BN lattice vibrations, which are not affected by die H/D exchange at the surface. Boron nitride powder is used commercially for lubrication. Its hexagonal lattice, with planar B3N3 hexagons, resembles that of graphite.

A close comparison of the FTIR spectra from gallium nitride GaN nanoparticies illustrated in Fig. 8.6 with the boron nitride nanoparticle spectra of Fig. 8.5 show how die various chemical groups -OH, —NH2, and -NH and their deuterated analogues have similar vibrational frequencies, but these frequencies are not precisely die same. For example, the frequency of the B-ND2 spectral line of Fig. 8.5 is somewhat lower than that of the Ga—ND2 line of Fig. 8.6, a small shift that results from their somewhat different chemical environments. The H/D exchange results of Fig. 8.6 show that all of the Ga-OH and Ga—NH2 are on the surface, while only some of the NH groups were exchanged. Notice that the strong GaH absorption band near 21,000 cm-1 was not appreciably disturbed by the H/D exchange, suggesting that it arises from hydrogen atoms bound to gallium inside the bulk.

Figure 8.S. FTIR spectra of borori nitride nanopowder surfaces after activation at 875 K (tracing a), after subsequent deuteration (tracing b), and (c) difference spectrum of a subtracted from b (tracing c). [From M.-l. Baraton and L Merhari, P. Quintard, V. Lorezenvilfi, Langmuir, 9,1486 (1993).]

An example that demonstrates the power of infrared spectroscopy to elucidate surface features of nanomaterials is the study of y-alumina (AI2O3), a catalytic material that can have a large surface area, up to 200-300 m2/g, due to its highly porous morphology. It has a defect spinel structure, and its large oxygen atoms form a tetragonally distorted face-centered cubic lattice (see Section 2.1.2). There are one octahedral (VI) and two tetrahedral (IV) sites per oxygen atom in die lattice, and aluminum ions located at these sites are designated by the notations viAl3+ and ivAl3+, respectively, in Fig. 8.7. There are a total of five configurations assumed by adsorbed hydroxyl groups that bond to aluminum ions at the surface, and these are sketched in the figure. The first two, types la and lb, involve the simple cases of OH bonded to tetrahedrally and octahedrally coordinated aluminum ions, respectively. The remaining three cases involve the hydroxyl radical bound simultaneously to two or three adjacent trivalent aluminum ions. The frequency shifts assigned to these five surface species, which are listed in the figure (v(OH)), are easily distinguished by infrared spectroscopy.

The FTTR spectra from y-alumina nanopowder before and after deuteration presented in Fig. 8.8 dispfay broad absorption bands with structure arising from

Figure 8.6. FTIR reflection spectra of gallium nitride nanopowder surfaces after activation at 500°C (tracing a), after subsequent deuteration (tracing b), and difference spectrum of a subtracted from b (tracing c). [From M.-l. Baraton, G. Carlson, and K. E. Gonsalves, Mater. Sci. Eng. B50, 42 (1997) .]

Figure 8.6. FTIR reflection spectra of gallium nitride nanopowder surfaces after activation at 500°C (tracing a), after subsequent deuteration (tracing b), and difference spectrum of a subtracted from b (tracing c). [From M.-l. Baraton, G. Carlson, and K. E. Gonsalves, Mater. Sci. Eng. B50, 42 (1997) .]

the OH and OD groups, respectively, on alumina activated at the temperature 600°C, and spectrum (a) of Fig. 8.9 provides an expanded view of the OD region of Fig. 8.8 for y-alumina activated at 500°C. The analysis of the positions and relative amplitudes of component lines of these spectra provide information on the distribution of aluminum ions in the octahedral and tetrahedral sites of the atomic layer at

IVAI* vi Al^ Type la Type lb v(OH) > 3750 cm"'

Figure 8.7. Five possible configurations of adsorbed hydroxyl groups bonded at tetrahedral (,vAI3+) and octahedral (V|AI3+) sites of a 7-alumina surface. [From M.-l. Baraton, In Nalwa (2000), Vol. 2. Chapter 2, p. 116.]

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