The group of Lab Preparation and Characterization of materials is specialized in the X-ray diffraction techniques (especially in X-rays multiple diffraction).
The X-ray diffraction is an important technique for the study of materials that provides information on the detailed atomic and molecular structure of several of them (notably the crystalline). It is possible with the X-rays diffraction methods, identify the atomic positions and chemical composition of the samples investigated, identify the crystalline phases present in it, as well as, the residual strains in their crystal lattice.
In the diffraction phenomenon, an X-ray beam focus on the material to be studied and it is detected the beam which emerges from it. By interacting with the atomic structure of the sample, the beam is diffracted by the atoms (inner electrons). As a result, several directions of the X-rays that emerge from the various materials carry information about the atomic structure of the same which are of major importance for its complete characterization. These data can be extracted and interpreted by analyzing the emerging X-rays. Thus, it is possible, after a detailed study to construct an image of the crystal structure using the positions of their atoms.
In the multiple diffraction phenomenon, in turn, a single X-rays incident beam simultaneously produces more than one diffracted beam in a variety of directions. The X-rays multiple diffraction (XRMD) is interesting because of its high sensitivity - it allows the detection of small distortions in the crystal lattice of the material. Also provides information on the three-dimensional networks, including on the possible interfaces between two distinct materials or between two regions with different crystal structures by using a special multiple diffraction case – the Bragg-Surface Diffraction (BSD). Thus, the method is able to extract structural information simultaneously from two or more crystalline regions within the samples, which usually is not available from other conventional techniques.
It is interesting to note that the information can be obtained both in the sample surface and the interfaces between different parts of it, when present. For example, one can determine the phase of the analyzed structure, allowing to distinguish between a structure and its "image" in the mirror. This important information is lost in the most traditional diffraction techniques.
The main part of the experimental group activities is performed on XRD1beamline the National Synchrotron Light Source (LNLS) in Campinas, which was adapted to the XRMD experiments. The reason is that the LNLS synchrotron radiation provides polarized X-rays on one plane, with high intensity, very low divergence and tuning the appropriate wavelength - conditions which, when taken simultaneously, allow for unique experiences with this technique. Furthermore, it is possible to change the polarization of the incidence plane in XRD1 beamline, which has turned out to be very important for some of the experiments with XRMD.
Main lines of research
Analysis of structural changes in crystals – Modifications may occur by crystal growth or by the action of external agents, such as variations of temperature, applied electric field on the sample, applying a mechanical stress on it, etc..
Study of piezoelectricity in crystals – so-called piezoelectric materials when they are subjected to a strain (or an external electric field) can produce a voltage (or a shift of their electrical charges - deformation of the crystal lattice).These materials appear on multiple devices in the everyday, such as certain types of speakers, microphones and lighters.
XRMD allows obtaining the piezoelectric coefficient of a single crystal, which is a parameter which characterizes how much it develops voltage at a given pressure. To use the XRMD in order to study these factors, the inverse piezoelectric effect is used instead of the direct effect. In this case, an applied electric field produces subtle deformations along distinct directions of the single crystal lattice. These microdeformations can be detected by multiple diffraction; from it may be inferred all coefficients in a very versatile way. In particular, the group has already obtained piezoelectric coefficients of various organic materials, as well as aminoacids singlecrystals.
Characterization of epitaxial semiconductor structures – epitaxial structures of a crystalline material are differentiated parts thereof situated close to the surface formed during some treatment processes. An example appears in the case of iron ions implanted in the silicon lattice, discussed below. As the semiconductor crystalline materials which exhibit high perfection, the X-rays multiple diffraction is very appropriate to study them in detail.
With the diffraction of multiple networks of epitaxial layers and the substrate are simultaneously analyzed, as well as the interfaces present in the sample. Information on the structural properties of epitaxial structures, such as strains in networks of epitaxial layer and substrate, both in the growth direction (perpendicular) and in the parallel direction (in-plane) can be simultaneously obtained. In addition, information on the stress distribution on the surface or interface, can be obtained from measurements directly using the mapping of surface-Bragg reflections (BSD), in which, the beam is diffracted along the substrate and/or epitaxial layer surface or even, from the interface between them.
Instrumentation for X-rays diffraction – This term refers to a sector of activity of research in Physics is the development of the necessary equipment to the laboratory. As the cutting-edge research and investigate new situations from new angles, in most times you can not find in the trade the necessary appliances. It is therefore necessary to build them or modify them within each laboratory.
The group has been dedicated to the construction and adaptation of instruments for X-rays diffraction to be used not only by the team itself as well as other IFGW and even the other institutions. In the Daresbury Laboratory of synchrotron radiation, UK, for example, the station 7.6, primarily designed for experiments X-ray topography was modified to allow for experiences using X-rays multiple diffraction through a collaboration between groups of LPCM in UNICAMP and University of Strathclyde in Glasgow, Scotland.
Multiuser Laboratory – Researchers can analyze the LPCM with the techniques and equipment at their disposal, samples sent by other research groups for this purpose, as well as receive students from other groups for experiments in collaboration with the laboratory. LPCM are also held through the provision of consulting services for companies seeking for X-rays expertise.
The latest research
Since 2008, the LPCM develops research in the areas of X-ray diffraction (XRD) and thin films (FF) synthesized by plasma of glow discharges.
X-ray Diffraction – In this area, the majority of publications focuses on applications of multiple diffraction as a high-resolution microprobe for the study:
- the effects of electric field in single crystals, allowing the measurement of piezoelectric coefficients;
- phase transitions to the change in temperature or electrical field;
- of semiconductor heterostructures, with studies of imperfections in the crystal structure and surface tension of these interfaces or heterostructures;
- the effect of impurities in the single crystal lattice, and
- Structural studies of semiconductor samples subjected to ion implantation process (with the process, ions can be implanted in a layer just beneath the surface of the material).In particular, shallow junctions are investigated and the formation of nanoparticles.
Other contributions of the group are performed to analyze the structural properties of three-dimensional nanometric materials, alloys and intermetallic compounds that exhibit the magnetocaloric effect.
Thin films – The activities in this area include dielectric and magnetic films. In the dielectric films, most of the publications concentrate on thin films of polymers produced by the technique of chemical vapor deposition by plasma assisted (PECVD, "plasma enhanced chemical vapor deposition"), are investigated in which the composition, molecular structure and properties optical, electrical and mechanical. Are also made studies on the modification of these properties by ion irradiation.
The investigations in magnetic films mainly focus on nanostructured thin films, in order to study the influence of the characteristic dimensions of nanostruturas in the magnetic properties and magnetocaloric effect of the films.
The example of the study of iron ions implanted in silicon
It will be reviewed here a detailed example of recent research conducted by the group, to illustrate the methods, procedures and concepts involved in this typical study. The example was withdran from Lang et al. Cryst. Growth Des. 10, 4363 (2010).
In this case, a study was done on iron ions (Fe+) implanted in silicon - Si (001). In sample preparation, FeSi2 metal nanoparticles have been formed into the sample, close to its surface during an epitaxial crystallization process called ion-beam-induced epitaxial crystallization (IBIEC). In order to study the formed nanoparticles it was used a special case of the XRMD called Bragg-Surface Diffraction (BSD), in which one of the two diffracted beams emerges parallel to the surface of the sample (or to an interface, when present). In the figures on the left are shown the incident ray (000) and the two diffracted, called primary (002) and secondary (111), the latter being parallel to the surface.
The three figures below give an example of analyzed parameters for this type of study.
History of the group
Research in Crystallography in IFGW were initiated by Professor Stephenson Caticha-Ellis (1930-2003), born in the small city of Melo, Uruguay. Caticha studied at Glasgow University and the Cavendish Laboratory at Cambridge University (UK) and did internships at the University of Paris (France), the Georgia Institute of Technology and Polytechnic Institute of Brooklyn (USA). He arrived in Brazil in the late 1960s, where he worked first at the Institute of Atomic Energy of St. Paul (now, IPEN, Institute of Energy and Nuclear Research), especially with neutron diffraction. In the early 1970s, he moved to the Institute of Physics Gleb Wataghin Unicamp, this time using X-ray diffraction.
Professors Stephenson Caticha Ellis and Lisandro Pavie Cardoso in 1984 photo at the door of LPCM
A group formed then returned to the study of the crystal defects, particularly using the multiple diffraction X-rays, which was an expert using the bases of the kinematics and dynamics of diffraction X-rays. Thus was created the group of Crystallography of the Institute of Physics Gleb Wataghin, which had the important contribution of several other researchers who passed through it.
After Caticha retired in 1991, with the split of the group, one of the teams - the X-ray diffraction Laboratory (LDRX today LPCM, whose activities are described in this text) - continued to explore the potential of multiple diffraction of X-rays and associated techniques particularly for single crystals, thin films and semiconductor epitaxial structures.
From 2008, the LPCM started developing investigations in X-ray diffraction and thin films synthesized by plasma of glow discharges, described in the body of this text.