Consortium
INCDFM,   NATIONAL INSTITUTE OF MATERIALS PHYSICS, BUCHAREST
ICF,             INSTITUTE OF PHYSICAL CHEMISTRY OF THE ROMANIAN ACADEMY
ICPE-CA     NATIONAL RESEARCH-DEVELOPMENT INSTITUTE FOR ELECTRIC ENGINEERING

NATIONAL INSTITUTE OF MATERIALS PHYSICS, BUCHAREST
INCDFM   http://www.infim.ro
Project Director:  Dr. Ioan Baltog  <ibaltog@infim.ro>

INSTITUTE OF PHYSICAL CHEMISTRY OF THE ROMANIAN ACADEMY
ICF   http://www.icf.ro/
Responsible: Dr. V. Fruth <vfruth@icf.ro>

NATIONAL RESEARCH-DEVELOPMENT INSTITUTE FOR ELECTRIC ENGINEERING
ICPE-CA   http://www.icpe-ca.ro/
Responsible: Dr. Ana Maria Bondar <abondar@icpe-ca.ro>

Methods & Laboratory Technologies

The first part of the project is focused on the PbI₂ crystal as reference bi-dimensional material with semiconducting properties. Ulterior, the investigations have been extended to other semiconducting layered materials. Thus, investigating the characteristics of the PbI2 microcrystal synthesized in the presence and in the absence of a polymer in the reaction medium , the following conclusions one obtained:
i) always, two types of particles, with different vibrational and photoluminescent properties (PbI₂ hexagonal platelets and KPbI₃ rods), are generated changing the stoichiometry of the synthesis reaction, which is achieved using water or acetonitrileas buffer solvent.
ii) the chemical reaction between KI and Pb(NO₃)₂ in the presence of polyacrylamide and polyethylene glycol in aqueous solution leads to the formation of exfoliated PbI2 structures ; iii) a slow quenching at the room temperature of a hot aqueous PbI₂ saturated solution containing a water-soluble polymer (polyacrylamide or poly (vinyl alcohol) ) has as result the formation of rods type structures; iv) the experimental results have demonstrated that regardless of the preparation method, the rods may be considered as typical morphological form of intercalated PbI₂ structure, whose specific signature is given by the Raman and photoluminescence spectra; v) Raman spectra show an orthorhombic structure formed through the intercalation between the PbI₂ layers of foreign molecular species. The intercalation activates through compression a supplementary vibration mode associated to longitudinal acoustic phonon noticed in a Raman spectrum by a band situated at 30 cm-1.Such a band resulting from a compression effect on I-Pb-I layer was anticipated by theoretical calculus; vi) Due to the compression effect produced by the penetration of foreign molecules between iodine layers, the hybridized electronic level situated at the top of the valence band, formed by the contribution of the 6s and 5p states of Pb₂⁺ and I⁻ ions, undergoes a deformation, by which the share of the 5p states of I⁻ ions is reduced.
Finally, the PL process into intercalated PbI₂ acquires all the characteristics of luminescence due to an intra-ion transition. Thus, PL is clearly observed when the Pb₂⁺ ions are dissolved in an alkali halide crystal. This transformation is evidenced by a wide intense emission band situated at 2.23 eV (band I) noticed at 77 K under λexc = 340 nm, considered the specific signature in PbI₂ intercalated PL spectra.
The lifetime of microseconds order revealed of this band is considered as being an convincing argument that by intercalation the basic semiconducting properties of PbI₂ are more changed.
The insertion between I-Pb-I layers of molecules containing atoms with pairs of non-bonding electrons (as ammonia and pyridine) has as result the modification of the crystal semiconducting properties. Exposing the crystal in ammonia vapors, the fundamental absorption egde of PbI₂ shifts towards higher energies, from 2.48 eV to 3.17 eV, which may be associated with a quantum confinement effect related with the nanometric size of the layers packages 4
formed during the intercalation process. If the exposure is made in pyridine vapors, in the PbI2 absorption spectrum is identified a new absorption band at 3.34 eV that is associated with the formation of PbI₂/pyridine complex. The characteristic signature into the PL spectra of intercalated PbI₂ with ammonia or pyridine consists in the appearance at 77 K of a green emission band when the excitation is made at higher energy (λexc = 335 nm) than the fundamental absorption egde (~500 nm). By intercalation of PbI2 with ammonia or pyridine molecules an expansion of the crystalline lattice along to the axis c takes place fact sustained also by the appearance of a supplementary maximum at smaler angles in the X rays diffraction spectra. In case of ammonia intercalated PbI2 ,the thermo-gravimetric data reveal the presence of two phases , one of them being unstable. The stable phase is characterized by a PbI2/(NH3) stoichiometric ratio of 0.85 . In the case of pyridine intercalated PbI2 the stable phase persists until 80 0C with a stoichiometric ratio of about 2 . The pyridine intercalation process between PbI2 layers is more complex, the additional compression effect due to the pyridine molecules diffusion into van der Waals space has as result the formation of a coordinative complex between PbI2 and pyridine molecules.
The intercalation of other semiconducting layer structure (BiI3, CdI2, CdCl2 and AgI ) with the same molecules, ammonia and pyridine, led to the following results: i) the intercalation of BiI3 and CdI2 occurs in two phases: at the first, the guest molecules diffuse in the van der Waals space and afterwards the metallic ions, i.e., Bi3+ and Cd2+ interact with the nitrogen atoms from ammonia or pyridine molecule. In the first phase a quantum confinement effect appears that is noticed by a shift towards higher energy of the fundamental absorption band of BiI3 and CdI2. In the second stage the appearance of new absorption bands indicate the coordinative complexes that are formed by the interaction between the two components. For the pyridine intercalation, the Raman spectra indicate the presence in two forms of this molecule between the two I-Pb-I two crystals layers: one resulting from a physical adsorption process, modeled through weak van der Waals bonds, and another one issued from a chemical adsorption process ended by the formation of coordinative complexes based on the chemical bonds.
The maxima that appear at small angles into the X rays diffraction spectra of BiI3 and CdI2 films confirms the insertion of guest molecules into the host crystalline lattice. The electronic microscopy reveals also that BiI3 and CdI2 platelets expand along c axis after the intercalation. The foreign molecules penetrated into BiI3 and CdI2 lattice determine the appearance of endothermic effects identified both in inert atmosphere as in the air in the thermogravimetry curves. For BiI3 the presence of two endothermic effects until 200 0C is owed to the intercalation substance binding mode: the first adsorbed through weak van der Waals bonds; the second adsorbed through strong forces in the BiI3 structure. Regarding CdI2, both ammonia as well as pyridine determine the appearance of endothermic effects caused by expulsion and decomposition reactions. The materials are stable until about 100 ºC and as commune characteristic they presents, over 200 ºC, CdI2 decomposition reactions into metallic cadmium and iodine (224 ºC for the ammonia system and 266 ºC for pyridine system).After the air treatment, both components are present, cadmium oxidizing at about 325ºC forms at CdO; ii) similar changes are observed for ammonia or pyridine intercalation between CdCl2 layers; iii) regarding AgI, the ammonia intercalation did not modify the optical, structural or thermic properties of the host material, fact that plead for a physical adsorption.
In order to understand better the organic conducting polymer intercalation process between inorganic semiconducting layers, we studied the PbI2/poly aniline hybrid material obtained through: i) electrochemical polymerization through cyclic voltametry of aniline on PbI2 crystal and ii) poly aniline with PbI2 mechanochemical reaction. The results are followings: i) using those two methods of preparation, a PbI2/PANI-ES (polyaniline – emeraldine – salt) material was obtained; ii) the intercalation process of PbI2 crystal with PANI-ES electrochemical obtained is confirmed in the Raman spectra through new Raman lines at 144 and 170 cm-1, which in our opinion are the result of steric hindrance effects; iii) PbI2 intercalated with aniline or polyaniline has as result the transformation of the PbI2 platelets into rods-like structure, fact that proves the forming of a new (PbI2)(C6H5NH2) compose that has a different crystalline configuration then the PbI2; iv) PbI2 and PANI-EB (poly aniline – emeraldine – base) mechanochemical interaction led to an acid pseudo-protonic doping of the semiconducting polymer; v) we presented the generation mechanism of certain luminescence bands engendered through photo carriers – electron - hole recombining localized on surface catchers generated through the PbI2 particles dimensional reduction – it is relieved for the first time a partial or total collecting process of electrons and holes through a charge transfer mechanisms towards a semiconducting (PANI-EB) or conducting (PANI-ES) material.
Another objective of the project was evidencing a quantum confinement processes of “quantum wells” type expected to be observed in semiconductors with layer structure. A priori, in the layered crystalline materials the “quantum wells” systems may be obtained breaking the continuity of atomic layers staking along c axe through the interposition of “stacking fault” type defects. In PbI2 crystal, these type of defects can be generated by quenching thermal treatment (QTT) that consist in the heating of sample in vacuum at 500K and its abrupt cooling at liquid nitrogen temperature (LNT). In this way are formed atomic I-Pb-I layer packages, having different nanometric thickness that behaves as quantum wells. In order to relieve the “quantum wells” structures, correlated PL and Raman scattering studies in polarized light were performed. The results are followings : i) the luminescence spectra of a PbI2 crystalline sample revealed three emission bands labeled E band (2.49 eV) associated to excitonic emission, D band (2.4 eV) dependent on volume defects and G band (2 eV) dependent on surface defects; ii) the luminescence spectra of thermal annealed crystals recorded in two configuration of polarization of the excitation light : s polarization ( E perpendicular on the measuring plane) and p polarization ( E parallel with measuring plane) reveal a D band (2.4 eV) that behaves differently: isotropically when c crystalline axis is contained into the measurement plane and anisotropically when c axis is perpendicular on measurement plane. For the last case, D band intensity is higher for p polarizing of excitation radiation.The result argues that the D band is associated to “stacking faults” defects induced in PbI2 crystal.
A result quite new is the excitation spectrum of the G band (2 eV) observed after a QTT in two polarization states of incident radiation. When p polarization light is used , one observe a fine structure formed from many emission bands, while for s polarization light only two maxima are detected. The anisotropic behavior of the excitation spectrum of G band demonstrates that a QTT applied to PbI2 generates I-Pb-I layer structures of nanometric thickness that are equivalent to “quantum wells” systems. The fine structure noticed in p excitation spectrum coincide with the fine structure associated with quantum confinement effects noticed in PbI2 thin films of different thickness i.e., two ,three, four layers of I-Pb-I.
A supporting point in this interpretation is furnished by the Raman spectra performed under resonant light excitation. The Raman spectra of PbI2 crystal, before and after QTT, recorded at room temperature (RT) and LNT in two different measuring geometries under excitation by light of 676.4 ( non-resonance) and 514.5 nm (resonance) disclose significant and subtle modifications. In the p configuration, achieved experimentally by the use of a half-wave-plate that changes the polarization from s to p, the overtone band is less intense. This indicates a weaker resonance, so that matching of the exciting light with the band edge absorption is no longer given. The explanation is immediate; the PbI2 structures resulting from thermal annealing are characterized by a wider band gap, so that p polarized excitation light of 514.5 nm no longer fulfills the resonance condition. The variation of the overtone band intensity is in the same sense as in the case when the as-grown PbI2 crystal is cooled to LNT, when the band gap also increased. It is for the first time when resonant polarized Raman spectroscopy is used to evidence the existence of “quantum wells” effects in a layered bulk material.

International Projects  proposed/accepted

The results of this project were obtained in the frame of the bilateral scientific cooperation established between the National Institute of Materials Physics, Bucharest Romania and the Institut des Materiaux “Jean Rouxel” Nantes, France

Conferences & Workshops

1. Raman and photoluminescence studies on low-dimensional PbI2 particles embedded in
polymer matrix
N. Preda, L. Mihut, M. Baibarac, I. Baltog

The Fifth International Edition of Romanian Conference on Advanced Materials, ROCAM, September 11-14, Bucharest-Magurele, Romania, 2006
2. Raman and photoluminescence studies on intercalated lead iodide with pyridine and iodine
N. Preda, L. Mihut, M. Baibarac, M. Husanu, C. Bucur, I. Baltog
8 th International Balkan Workshop on Applied Physics, July 5-7 th, 2007, Constanta,
Romania

3. Films and crystalline powder of PbI2 intercalated with ammonia and pyridine
N.Preda, L.Mihut, M.Baibarac, I.Baltog, R.Ramer, C.Andronescu, V.Fruth
The International Conference on Optical Optoelectronic and Photonic Materials and
Applications, 30 July-3 August 2007, London, UK

4. Raman and photoluminescence studies on intercalated lead iodide with pyridine and iodine
N. Preda, L. Mihut, M. Baibarac, M. Husanu, C. Bucur, I. Baltog
8th International Balkan Workshop on Applied Physics, Constanta, July 5-7, 2007, Romania

5. Intercalation of layered metal iodides with pyridine evidenced by Raman spectroscopy
N. Preda, L. Mihut, M. Baibarac, I. Baltog
European Materials Research Society , Warsava, sept 2008, Poland

Published Papers

1. Optical properties of low-dimensional PbI2 particles embedded in polyacrylamide matrix
N. Preda, L. Mihut, I. Baltog, T. Velula, V. Teodorescu
Journal of Optoelectronics and Advanced Materials, 8, 909-913, 2006

2. A distinctive signature in the Raman and photoluminescence spectra of intercalated PbI2
N. Preda, L. Mihut, M. Baibarac, I. Baltog, S. Lefrant
Journal of Physics: Condensed Matter, 18, 8899-8912, 2006

3. Raman and photoluminescence studies on low-dimensional PbI2 particles embedded in
polymer matrix
N. Preda, L. Mihut, M. Baibarac, I. Baltog
Journal of Optoelectronics and Advanced Materials, 9, 1358, 2007

4. Raman and photoluminescence studies on intercalated lead iodide with pyridine and iodine
N. Preda, L. Mihut, M. Baibarac, M. Husanu, C. Bucur, I. Baltog
Journal of Optoelectronics and Advanced Materials, 10, 319, 2008

5. Photoluminescence and Raman spectroscopy studies on polyaniline/PbI2 composite
M. Baibarac , I.Baltog , S.Lefrant
Journal of Solid State Chemistry 182 (2009) 827–835, (2009)

6. Films and crystalline powder of PbI2 intercalated with ammonia and pyridine
N.Preda, L.Mihut, M.Baibarac, I.Baltog, R.Ramer, J. Pandele, C.Andronescu, V.Fruth
J. Mater. Sci.: Mater. Electron., 20, 465-470,2009

7. Quantum well effect in bulk PbI2 crystals revealed by the anisotropy of photoluminescence and Raman spectra
I Baltog, M Baibarac1 and S Lefrant
J. Phys.: Condens. Matter 21 , 025507 (9pp), (2009)

Resources