Physics, Chemistry a n d Application of Nanostructures Reviews and Short Notes to Nanomeeting 2003 # *« 1 Editors V E Borisenko S V Gaponenko V S Gurin World Scientific Physics, Chemistry a n d Application of Nanostructures Reviews and Short Notes to Nanomeeting 2003 This page is intentionally left blank Physics, Chemistry a n d Application of Nanostructures Reviews and Short Notes to Nanomeeting 2003 Minsk, Belarus 20 - 23 May 2003 Editors V E Borisenko Belarusian State University of Informatics and Radioelectronics, Belarus S V Gaponenko Institute of Molecular and Atomic Physics, Belarus V S Gurin Belarusian State University, Belarus V | f e World Scientific wb New Jersey • London • Singapore • Hong Kong Published by World Scientific Publishing Co Pte Ltd Toh Tuck Link, Singapore 596224 USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES Reviews and Short Notes to Nanomeeting 2003 Copyright © 2003 by World Scientific Publishing Co Pte Ltd All rights reserved This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA In this case permission to photocopy is not required from the publisher ISBN 981-238-381-6 Printed in Singapore by World Scientific Printers (S) Pte Ltd INTERNATIONAL CONFERENCE HANOMEEVNG-200Z Minsk, Belarus, May 20-23, 2003 ORGANIZERS Belarusian State University of Informatics and Radioelectronics (Minsk, Belarus) and Universite de la Mediterranee Aix-Marseille II (Marseille, France) v INTERNATIONAL ORGANIZING COMMITTEE V E Borisenko - Co-chairman F Arnaud d'Avitaya- Co-chairman L J Balk E V Buzaneva J Derrien S V Gaponenko B W Licznerski L W Molenkamp H Morisaki A Nassiopoulou S Ossicini K A Valiev (Belarus) (France) (Germany) (Ukraine) (France) (Belarus) (Poland) (Germany) (Japan) (Greece) (Italy) (Russia) BELARUSIAN NATIONAL ORGANIZING COMMITTEE P I Brigadin - Chairman M P Batura V E Borisenko V S Gurin L I Ivanenko F F Komarov V A Labunov A A Leshok V V Nelaev VI FOREWORD The first years of the XXI-st century have brought new fundamental knowledge and novel applications of nanostructures Nanoelectronics and nanophotonics, bioinformatics and molecular electronics are extensively progressing on the basis of recent achievements in nanotechnology The results obtained are discussed at NMOMeemc;-2001 (20-23 May, 2003), which is the International Conference on Physics, Chemistry and Application of Nanostructures traditionally organized each two years in Minsk (Belarus) The book that you keep in your hands collects invited reviews and short notes of contributions to NANOMEEWG-2001 The papers in the book are arranged in traditional sections: Physics of Nanostructures, Chemistry of Nanostructures, Nanotechnology and Nanostructure Based Devices Both basic and applied researches are presented Among different results characterizing our knowledge about the nanoworld, one can note an increased interest to Ge/Si quantum dot systems, photonic crystals, carbon nanostructures, biological molecules, self-scrolled semiconductors, epitaxial GaAIN onto Si Their indeed astonishing properties promise a birth of novel approaches to information processing Scanning probe techniques and nanochemistry, self-organization and self-assembling have got new i mpetus to be applied in nanotechnology The examples can be found in the book The style of the presentations has been mainly preserved in its original form We deeply acknowledge Sponsors provided the financial support for the Conference Victor E Borisenko Francois Arnaud dAvitaya Minsk and Marseille January 2003 Co-chairmen of NANOMEE11NC;-2001 VII This page is intentionally left blank CONTENTS Foreword vii PHYSICS OF NANOSTRUCTURES Si/SiGe nanostructures: challenges and future perspectives (invited) D Grutzmacher Spin resolved inverse photoemission from layered magnetic nanostructures (invited) R Bertacco, L Dud, M Marcon, M Portalupi, F Ciccacci Nonlinear optical properties of one-dimensional photonic crystals (invited) C Sibilia, G D 'aguanno, M Centini, M C Larciprete, M Bertolotti, M Scalora, M Bloemer 11 19 Tunable three-dimensional photonic crystals based on opal-V0 composites (invited) V G Golubev 24 Interband transitions in Si nanostructures within effective mass approximation (invited) X Zianni, A G Nassiopoulou 32 Photoluminescence of Er 3+ ions in opal/tellurite glass composite nanostructures A V Gur'yanov, M I Samoilovich, M Yu Tsvetkov, E B Intushin, Yu I Chigirinskii 39 Time-resolved luminescence of europium complexes in bulk and nanostuctured dielectric media E P Petrov, D A Ksenzov, T A Pavich, M I Samoilovich, A V Gur'yanov Synchrotron investigations of electron-energy spectra in silicon nanostructures E P Domashevskaya, V A Terekhov, V M Kashkarov, E Yu Manukovskii, S Yu Turishchev, S L Molodtsov, D V Vyalikh, A F Khokhlov, A I Mashin, V G Shengurov, S P Svetlov, V Yu Chalkov IX 43 47 PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2003 INVITED RELAXATION PROCESSES IN RARE EARTH DOPED CRYSTALS AS STUDIED BY HIGH RESOLUTION FOURIER SPECTROSCOPY M N POPOVA Institute of Spectroscopy, RAS, 142190 Troitsk, Moscow Region, Russia E-mail: popova® isan, troitsk ru B Z MALKIN Kazan State University, 420008 Kazan, E-mail: Russia boris.malkin@ksu.ru Experimental method and the theoretical approach developed to study relaxation processes in rare-earth doped crystals are briefly outlined on the examples of Er3+:liYF4 and Pr^CsCdBn Possible applications to nanostructured materials are discussed Introduction Thin film structures doped with luminescent lanthanides are of primary interest for optoelectronics In most cases, lanthanides enter solid state in the form of trivalent ions, Ln3+ Optical 4f electrons of Ln3+ ions are well shielded by the outer s2 and 5p6 filled electron shells, so that the lines of optical transitions remain relatively narrow even in condensed matter A reach energy spectrum of 4f configuration results in a possibility to observe absorption (emission) of Ln3+-doped solids in a wide spectral range extending from the far-infrared via visible to ultraviolet region In particular, the infrared luminescence of Er3+ at about 1.5 um (corresponding to the 4Ii3/2-*4Ii5/2 transition) falls into a maximum transparency window of optical fibers Much work has been done to incorporate lanthanides, Er in particular, into silicon-based optoelectronic materials Promising results on an intensive Erluminescence have been obtained from porous Si coated by sol-gel derived xerogel films (see, e.g., [1]) However, the tree-like nonregular configuration of pores in mesoporous silicon leads to a non-reproducible Er-luminescence intensity [1] The use of regular porous structures of anodic alumina [2] has allowed to considerably improve the parameters of luminescent Ln3+-containing films [3] Strong enhancement of Er and Tb luminescence from xerogel films confined in mesoporous anodic alumina has been reported recently [4] In contrast to lanthanide-doped semiconductor films, a strong reduction of concentration quenching was observed To further improve characteristics of lanthanide-based optoelectronic materials, it is important to study the nature of luminescent centers in them, the interaction of 560 optical electrons with the matrices (in particular, with atomic vibrations), the relaxation processes In the present paper we show that the method of optical Fourier-transform spectroscopy offers new possibilities in studying infrared transitions of Ln3+ ions, in comparison with other spectroscopic techniques (Section 2) Our research on Er3-1" in LiYF4 presents an example what information on relaxation processes can be extracted from the spectra (Section 3) An important effect of an essential redistribution of the phonon density of states in solids doped with lanthanide ions is discussed in connection with our work on Pr +:CsCdBr3 (Section 4) Fourier-transform spectroscopy 2.1 Fundamentals The principal parts of a Fourier-transform spectrometer (FTS) are the Michelson interferometer with a moving mirror (see Fig.l) and a computer that performs the Fourier transform of the interferogram and thus calculates the spectrum of an incident radiation E(t) A detector registrates the averaged intensity at the output of the Michelson interferometer 0(t) ~[E(t) + E(t +T)p = E2{t) + E2{t + t) + 2E(t)E(t + T) (1) Here r=l/c is the time delay between the V/////A two interfering beams, / being the path difference Under certain conditions, that usually are fulfilled, the fist two terms in (1) are equal to the time-independent mean intensity while the third term represents the autocorrelation function \ff(z) = E(t)E(t+T) (2) which depends on the time delay T The Figure Scheme of the Michelson Fourier transformation of y/(j) yields the interferometer spectrum, namely, B(co) = — jyf (r) cos cmdt K (3) A real instrument delivers ^(T) for the interval [0, L/c], where L is the maximum path difference which depends on the maximum displacement of the moving mirror of the FTS The L-dependent instrumental function, that is the response of an instrument to a monochromatic incident light E(t)-coscat)t, can be written as sin(o)~co0)L/c (4) h n (CO-COQ) 561 fi ->z,_»oo 5{(O-(OQ), where 8((0-(Qo) is the delta-function; for a finite L,fL exhibits secondary maxima, the distance between the first zeroes of fL, 8OO=2TK/L, or, in wavenumbers, 5a=l/L, characterizes the spectral resolution The free spectral range Aa for the FTS depends on a number N of points registered in the interferogram: Aa=N6a (5) Typically, AMO6, so even for high resolution (So- 10"3 cm'1) instruments Aa~ 103cm"1, and thus broad-band high-resolution spectroscopy is possible with FTS FTS analyses all the spectral elements simultaneously which results in the so called Fellgett or multiplex advantage of FTS over conventional instruments in signal-to-noise ratio For the intensity-independent noise of a detector (infrared spectral region) the Fellgett advantage is maximum, namely, , where M=Ao/8o is the number of registrated spectral elements Additional gain comes from a large throughput of a FTS in comparison with a slit spectrometer (Jacquinot advantage) In Fourier transform spectroscopy, the wavenumber scale is established relative to the frequency of a stabilized laser used for the measurement of the path difference and, as a result, is precise in the whole spectral region (Connes advantage) General advantages of the FTS over conventional instruments make them a useful tool for material research In particular, it is advantageous to register by FTS a weak absorption (as in the case of lanthanide ions in films or nanostructures) 2.2 Experimental setup In our research, we used a BOMEM DA3.002 Fourier-transform spectrometer equipped with a controlled-temperature (±0.1 K in the range between and 300 K) helium-vapor cryostat High-resolution (up to 0.005 cm"1) spectra were registered in the spectral range between 2000 and 11000 cm'1 Relaxation processes in Er3+:LiYF4 The scheelite crystal LiYF4 doped with Er3+ ion is well known as an efficient multifrequency laser material Laser actions at 2.7 and 1.5 um that occur between the I u / and 4I13/2 and, respectively, 4Ii3/2 and 4lis/2 multiplets have attracted particular interest because of applications in optical fiber lines and medicine Spectra corresponding to the 4lna~*"4Ii3/2» \m transitions are presented in Fig The main goal of our high-resolution work on Er3+:LiYF4 [5] was to compare the experimentally measured linewidths with the calculated ones and thus to check the validity of the previously developed theoretical model of the electron-phonon interaction [6] for calculation of phonon relaxation rates 562 0,8: c o,4H - \ • 0,0- « (a) 6600 c Figure Transmittance spectra of L1YF4: (0.2%) at K in the region of the transitions (a) /i5/2-A /2 and (b) 4/is/2—4/u/2- E , H l c polarization 6700 l.o-r 0,60,2- 10250 10300 W ave number (cm"1) The crystal-field Hamiltonian is represented by the expression knk l cf Hcf - X B„C P~P (6) P.k where C£ are irreducible tensor operators and £c are crystal-field parameters The electron-phonon interaction comes from the change of crystal-field parameters due to displacements of atoms from their equilibrium positions in the lattice In linear approximation it can be written as follows: H el-Ph = I V a ( j ) [ « a ( s ) - i ( a ( £ r ) ] ; Va(*)=X*J.a(*)C*, (7) pk where u(j)-u(Er) is the difference between dynamic displacements of the ligand ion s and the Er3+ ion, oc=.x:,y,z, and fl^ (5) are the coupling constants The latter can be calculated in the framework of a model giving an explicit expression of crystal-field parameters on distances between a Ln3+ ion and other ions in the lattice We use here the exchange charge model [6] to calculate the coupling constants and crystal-field levels and wavefunctions The probability of the one-phonon transition between the electronic initial (1) and final (/) states with energy gap /jfity>0 can be presented as %=T I ( / | V a ( ) l m ^ ( ^ l £ ) ( i l y j g ( / ) | / ) x [ n ( a ) l / ) + lJ (8) sas'P ' ' where n(co) is the phonon occupation number and gap(ss'\co) are the linear combinations of the lattice Green's functions for the differences between displacements of ligands and the Er3+ ion [5] We performed a calculation of the relaxation rates for all the crystal-field sublevels within the manifolds of 4In/2 and z 563 Ii3/2 multiplets Frequencies and polarization vectors of phonons in the LiYF4 crystal were obtained at 8000 points in the irreducible part of the Brillouin zone using the rigid ion model of lattice dynamics derived on the basis of neutron scattering data Matrix elements of electronic operators Va(s) were calculated with the wave functions obtained from the crystal-field calculation The inverse lifetimes of the crystal-field sublevels determine the widths of corresponding absorption lines l/r.=W.=1W (9) / Despite many simplifying approximations there is a good agreement between the measured linewidths and the estimated relaxation rates [5] (see Table 1) Redistribution of the phonon density of states in activated crystals: Pr^iCsCdBrs A further development of the displayed approach to the electron-phonon interaction and relaxation processes in crystals activated by lanthanide ions came from the high-resolution study of Pr3+:CsCdBr3 [7] These crystals have recently attracted a considerable interest being a promising material for up-conversion (a) lasers due to the property of their quasi-one% dimensional lattice to incorporate Ln3+ ions in pairs Resolved hyperfine structure (hfs) due to the interaction of optical electrons with the magnetic 6483.0 6483.3 6483.6 Wave number (cm") moment of the Pr nucleus (1=5/2) was observed in the spectra (Fig 3).The measured hyperfine splittings and Figure (a) Measured widths of hyperfine sublevels found from the hyperfine structure with resolution of 0.005 cm"1 experimental line shapes are presented in Table I and (b) calculated Even at the liquid-helium temperature the hfs of the hyperfine structure of the most of excited crystal-field sublevels is masked by the 3 // ( r )^ F3( r ) spontaneous relaxation broadening Strong electrontransition of CsCdBr3:Pr3+ phonon interaction effects in CsCdBr3:Ln3+ crystals originate from the specific density of phonon states that has large maxima in the low-frequency region (20-40 cm"1) in the perfect crystal lattice (see Fig and Ref [8]) We performed a calculation of the relaxation rates using the phonon Green's functions of the perfect (CsCdBr3) and locally perturbed (impurity dimer centers in CsCdBr3:Pr3+) crystal lattices obtained in Ref [8] The formation of a dimer leads to a strong perturbation of the crystal lattice (mass defects in the three adjacent Cd2+ sites and large changes of force constants) As it has been shown in Ref [8], the local spectral density of phonon states essentially redistributes and several localized modes appear near the boundary of the continuous phonon spectrum of the m (b) l^*^*w^' 564 Table Measured linewidths in the absorption spectra [5,7] and calculated one-phonon transition probabilities in UYF4:Er3+ and CsCdBr3:Pr3+ (T=5 K) CsCdBr3:Pr3+ UYF4:Er3+ Multiplet, CF level Multiplet, CF level r78 10312 2) E, cm"1 10306 0.76 0.41 r 6496 3.20 0.80 10295 0.21 0.44 r,a) 6483 0.75a 0.13 10277 0.20 0.76 r, 5153 52.8 30.9 r3(2) 5148 - 19.4 r ,o) H5r3 5073 0.94a(1.32b) 2620° 52.0 2593c - 2547 13.2 18.2 2332 62.2 48.7 2317 56.6 116.4 2261 5.65 (6.4") In/2 r78< r56 r 56 (2 > r "> r78 F2 0.007 10218 6738 7.0 6.22 E, cm"1 r 6724 2.58 2.7 r78 r,o> 4.11 6696 2.5 r56(3) 2.89 6672 2.0 r, r78< > r ,B) 0.10 0.07 6579 r56 r ,d) 0.0003 0.017 6538 r56(" r d) 6534 0.015 r78