Elemental and Chemical Composition of Organic/Inorganic Nanostructures

Elemental and Chemical Composition of Organic/Inorganic Nanostructures

Experimental Methods of Composition Study

All traditional methods of elemental and chemical composition study, such as XPS,Auger spectroscopy, Raman spectroscopy, and Furrier Transformed Infrared Spectroscopy(FTIR), were adopted for nanostructural materials [11, 17, 46]. Electronspectroscopy is based upon two schemes of interaction of either high-energy electronsor X-rays with electrons on core atomic levels (see Figure 3.16).In the first case [Figure 3.16(a)], an incident high energy electron beam maycause secondary electron emission from one of the core levels. The vacancy is immediatelyfilled with an electron from the higher level, and results in either the emissionof an electromagnetic wave in the X-ray spectral region (1), or the energy transfer toanother electron and its subsequent emission (2). The second event is called theAuger process. Both processes are exploited for the elemental analysis of thematerial [47].The spectrum of emitted X-rays provides a crude elemental analysis calledenergy dispersion X-ray (EDX) analysis. This technique goes along with standardSEM, and requires an additional X-ray detector. The accuracy of this method is notgreat, but it is sufficient to get a general idea of the material’s elemental compositionand stoicheometry. Another advantage of this technique, combined with electron

beam scanning, is the possibility of mapping the material by plotting twodimensionalimages formed by different elements. Similar information can beobtained with Auger electron spectroscopy (AES).The method of XPS, which is based on the registration of electron spectra emittedfrom core atomic levels as a result of X-ray irradiation gives amore accurate information on the material’s composition. The resolution of thespectra of secondary electrons emitted from the atomic levels is in the range of fractionsof electron volts, which is enough to distinguish between different chemicalforms of the same elements.Another excellent technique for chemical analysis is infrared spectroscopy (IR),which is based upon the registration of characteristic spectra of molecularvibrations in the materials. Such spectra can be measured with either conventionalIR, FTIR spectroscopy, or Raman spectroscopy. The latter is based on the registrationof vibration spectra excited by a powerful laser in the visible spectral range.Raman spectral shift between the main excitation line and coupled vibrationfrequencies is the characteristic parameter for the recognition of molecularvibrations. An extensive database exists for vibration frequencies of differentelements in different materials, which allows researchers to find chemical compositionfairy easily

Examples of Composition Study of Materials Prepared by Chemical Routes

Typical EDS graphs of sol-gel TiO2 films, as shown in Figure 3.17(a), demonstrate
the presence of titanium and oxygen; however, it is difficult to establish the stoichiometry
due to the interference of SiO2 and other oxides existing in the sol-gel film, as
well as in the glass substrates [35]. Glass is a complex compound, containing a
number of different elements (Si, Na, K, Mg, and Ca), as can be judged from EDS
data. TiO2 films deposited by sputtering on silicon wafers yield much more defined
spectra [see Figure 3.17(b)], practically without impurities, other than from silicon
coming from the substrate.
XPS confirmed the formation of the PbS phase in thin films of lead phthalocyanine
(PbPc) after exposure to H2S gas [49]. The XPS peaks of both Pb-4f and S-2p in
Figure 3.18(a) show a complex chemical composition of the material.

Control of Impurities in Chemically Deposited Nanostructures
The problem of the registration and control of low concentrations of impurities (onthe level of parts per million, or parts per billion) in organic materials is of highimportance. Since organic materials have become increasingly popular in nanoelectronicdevice applications, their purity must be comparable to that in inorganicmaterials. Unfortunately, this is not always the case. Even the use of high puritygrade (99.99 or 10−4) chemicals during the formation of nanostructures (e.g., nanoparticles,layered structures, or thin films) does not match the grade of semiconductormaterials in microelectronics, which is typically of 10−6 and higher. That explainswhy materials produced by chemical routes vary in their properties so much. Itrecalls, to a large extent, the situation in semiconductor material technology in1950s and 1960s, when the same types of materials produced in different laboratoriesshowed absolutely different electrical properties. Organic materials technologyis, however, progressing much faster because of great experience in materialsscience, advanced analytical techniques, and technologies of materials processingnow available.The importance of purity control of nanostructured materials has been recognized,and the research in this direction is constantly growing. It includes a number
the resolution limit of Auger and X-photoelectron spectroscopy, and of optical (luminescence)methods [46, 47]. An obvious conclusion is to deploy electrical methodsfor impurity control in nanostructured materials, particularly in nanoparticles. Electricalmethods may also be useful in monitoring the density of surface states, whichplay a significant role in the electrical properties of semiconductor nanoparticles.
Asignatura: E.E.S
Saithrhu R. Gonzalez C.