Physics - 113 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Theoretical Calculation of Reorientation Energy in Metal /Semiconductor Interface H. J. M. A-Agealy , H. Kh. Mujbi Department of Physics, College of Education Ibn Al-Haitham, University of Baghdad Receired in:19June 2011 : Accepted in: 20 September 2011 Abstract A theoretical calculation of the reorientation energy for non adiabatic electron transfer at interface between metal and semiconductor system was carried out. The continuum outer sphere theory of electron transfer reaction has been extensively used for electron transfer between metal/semiconductor interface .It is found that in these calculations the reorientation energy is proportional to the optical and statistical dielectric constant of semiconductor , properties of metal ,and the distance between metal and semiconductor .Results of reorientation energy show that ZnO semiconductor with metal Au possess a good matching as compared with ZnS and ZnSe . Theoretical calculation showed a good agreement with experimental value. Key words:- theoretical calculation , reorientation energy, metal semiconductor, metal/semiconductor, Gold(Au),ZnO,ZnS,and ZnSe semiconductors. Introduction Electron transfer (ET) reactions plays an important role in a huge number of chemical and biochemical reactions[1]. The field of electron transfer has a rich history and continues to pose interesting problems for today researchers. Electron transfer theory predicts reorganization energy to be one factor that determines the electrochemical potential of a meta lion [2]. Reorganization energy, is the energy required to change the structure of the reactants, or activate them, from that of equilibrium to that of the products. Reorganization energy can be divided into two parts: an inner sphere component, , and an outer sphere component,. The inner sphere component encompasses hanging bond lengths and geometries orientation, while the outer Spherion compasses intermolecular electronic interactions such as alignment of a polar system dipole moment. The pioneering studies of Marcus considered a classical description of the solvent fluctuations, which were subsequently handled in terms of a quantum-mechanical theory of solvent modes, developed by Lavish and his school. Marcus’ had also advanced a classical description of inner-sphere reorganization effects[3]. A quantum-mechanical treatment of non adiabatic ET processes is available, which provides a quantitative description of the effects of configurationally changes[4] The Marcus model of solvent reorganization represents the microscopic field of the acting on the transferred electron by a cavity field of a dielectric continuum. The model is very useful and provides many important insights into solvent effects on the activation barrier for ET and other types of reaction in polar liquids. It is exceptional to have a description of reorientation in terms of only two, the refractive index and the static dielectric constant. On the other hand, this concept suggests that ET reactions (and, more generally, reactions producing changes in multiple state), interact only with the macroscopic electric field in a polar medium. Clearly, the reaction proceeds on a molecular length scale and is coupled to specific molecular motions of the solvents well as to the macroscopic electric field[5].In our research we study Physics - 114 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 and calculate the reorientation energy for gold Au with ZnO ,ZnS, and ZnSe semiconductor interface. Theory Theoretical aspects of non adiabatic ET process are well established within the classical, semi classical, and quantum mechanical framework[6] .In all these theories the first step is the calculation of rate electron transfer which is the reorientation energy. The reorientation free energy is calculated for a reaction between a reactant and some semiconductors. We consider a no equilibrium system having some charge distribution, expressed in terms of equilibrium free energies, the free energy of formation[7]. = (λi + λo ) …….. (1) where is the free energy for reactant , is the free energy of product, λ, is the total energy of any vibration contribution of the reorientation energy from the reactants λi, due to changes in the equilibrium values of their vibration coordinates due to the reaction; λo, is given by [7] where , are the contripution of reorientation for semiconductor ,metal ,metal-semiconductor semiconductor-metal, and semiconductor metal contact. ….…….(3) , , , , , , , and, are the dielectric constant for semiconductor and metal,refrective index for semiconductor and metal ,distance between metal semiconductor, radii of semiconductor and metal and the distance for semiconductor and metal to electrode respectively. Result In this paper, our main theoretical calculation of reorientation energy in metal/semiconductor interface system .The tool is an outer inner sphere equation that given by Marcus.ET reactions from a no equilibrated initial state. Semiconductor /metal interface system provide interfacial charge transfer processes. To study and calculate the rate constant of electron transfer in Au/ZnO, Au/ZnS, and Au/ZnSe system, first we calculate the reorientation energy λ(eV) for these systems. To calculate the reorientation energy we extensively the relation of Hsu-Marcus[7] that derived upon continuum sphere for two materials system. The reorientation energy for gold metal interface with ZnO, ZnS., and ZnSe semiconductor system are calculated using Eq.( 3 ). Inserting the values of radii Rse= 3.8025A0,5.4100A,and 5.6600A0 [10] for ZnO,ZnS,and ZnSe semiconductor , Rm= 1.6600Ao for Gold metal [8] for metal and the distance dm=am+1, dse=ase+1, Rm-se=am+ase with the values of nse,nm, are refractive index of Physics - 115 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 semiconductor and metal and static dielectric of semiconductor and metal from table(2). Results have been summarized in table(3). Discussion As for the metal/semiconductor system interface a reorientation energy that is given to rearrangement of the conduction electrons occurs at the interface. The results of reorientation free energy for Au/ZnO, Au/ZnS and Au/ZnSe system indicate illatively weak coupling coefficient when the distance dm and dse increase with Rm and dse . These because of the barrier height in metal/semiconductor system large with increase of d, and have a reorientation free energy to arrangement system is large The value of λ(eV) observed large where a metal/semiconductor is not contact . This refers and investigates the barrier height in contact which is minimum. However, some electrons flow from the metal into the semiconductor because of potential height barrier and electrons accumulated at the interface instead of receding from it. The results of the Au/ZnO system possess a good matching as compared with Au/ZnS, and Au/ZnSe.Tabe (3) shows the results of the reorientation free energy for Au/ZnO system which is more effective to Au metal than ZnSe and ZnS for the same metal. This indicates that ZnO possess optical index smaller than ZnS and ZnSe also the crystal of structure is wurtzite[10], which is difference than ZnS and ZnSe have Zinc blend structure[10].On the other hand the energy gap of ZnO is about 3.4(eV)[10] with a better carrier of electron mobility, and electron affinity is larger than the other ZnS and ZnSe. This means that the density of electron in conduction band of ZnO is larger than the other semiconductor .It is leading to suggest that Au/ZnO is attraction and a good system obviously, a large value of reorientation energy in semiconductor /Au metal would give rise to large transfer of electron, however these value are good for three semiconductor but the ZnO is a better than two semiconductors . Our results are in good coincident with the other experimental and theoretical value in table(4). Conclusion When a metal is making intimate contact with a semiconductor , the reorganization energy in minimum, this indicates the Fermi levels in two materials must be coincident at equilibrium and that given which is arrangement of change carrier is suitable to transfer. In summary it can be conclusion depending on present results that the reorientation energy for change transfer across metal/semiconductor interface showed strong dependence on the type of semiconductor (properties of semiconductor). For increasing the distance between gold metal and ZnO, ZnS, and ZnSe semiconductor the reorientation free energy is increasing proportional with distance that's mean the height of barrier is large that is formed between two materials. We concluded from the present results that the reaction of electron transfer strongly depends on system. Large values of the reorientation energy in Au/ZnO system lead to suggest the ZnO is good matching reaction towards the gold meta as compared with other semiconductor ZnO and ZnSe. References 1. Anne,M.K.;Stephan,L. and Gu, G.,( 2001), Quantum mechanical investigation of the inner-spherereorganization energy of cyclooctatetraene/ cyclooctatetraeneradical anion, Spectrochimica, Acta Part A, 57 : 1959–1969. 2. Charlotte,A.;Mason,T.J.; Amanda L.; Eckermann, S. and Binding, (2005), Studiesof Solvent Reorganization Energy Probes, Nanoscape, 2(1): 97. Physics - 116 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 3. Ephraim, B.;Mordechai, B.;Joshua ,J.; and Navon ,G. (1981), Quantum Effects on the Rates of Electron-Transfer Reactions, J. Phys. Chem. 85, 3759-3762. 4. Kestner, N. R.; Logan; Jortner, J.,(1974), theory of electron transfer in liquid, J. Phy. Chem., 78, 214. 5. Peter, V.; Matthew, B.; Zimmt,D. V.;Matyushov, and Gregory, A. V. (1999), A Failure of Continuum Theory: Temperature Dependence of the Solvent Reorganization Energy of Electron Transfer in Highly Polar Solvents, J. Phys. Chem., 103, 9130-9140. 6. Gautam ,B.;Akio, K.; Atsuo, K. and Nobuhiro ,G. (1998), Protein Electron Transfer Reorganization Energy Spectrum from Normal Mode Analysis Theory, J. Phys. Chem ., 102, 2076-2084. 7. Marcus,R.A. (1989), Reorganization Free Energy for Electron Transfers at Liquid-Liquid and Dielectric Semiconductor-Liquid Interfaces, Chemistry California Institute of Technology, 58, 55-95. 8. Cutler, J.C, (2008), Hand Book,Washington, D.C, Environment information coalition council for science and the Environment. 9. Shashi,G. (2002), electron transfer at metal surfaces", ,Ph.D theses , California Institute of Technology, Pasadena, California 10. Ruzyllo, J. (2009), hand book semiconductor material, semiconductor glossary .com. 11. Edward, D. P. (1985) , Handbook of Optical Constants of Solids, Academic Press, Boston,. 12. Ashok. B. ;and Durger, B.,( 2010), photolumensens and photoconductivity of ZnS,ZnO coated nanowires, Appl.Mater. Interfaces, 2(2): 408-412. 13. Vos, M.; Xu, F.; Weaver, J. H. and Cheng, H., (1988), Influence of metal interlayers on Schottky barrier formation for Au/ZnSe (100) and Al/ZnSe (100), Applied Physics Letters , 53(16): 1530 -1532. 14. Marcus, R. A. and Sutin, N. (1985), Electron transfer in chemistry and Biolog, Biochim. et Biophys , Acta 811, 265. Table(1): Properties of metals Gold(Au) Metal Properties 196.97[8] Atomic weigth 10.2[8] Atomic volume(cm3/mol) 144 [8] ِ◌Atomic radius(pm) 1.658[11] Refractive index Cubic face centerd Crystal structure 4.080 [8] Lattice constant(A0) 5.1 [8] Electron work function(ev) 19.32[8] Density(g/cm3) 7.32[9] Fermi energy(eV) http://dx.doi.org/10.1063/1.99947 http://dx.doi.org/10.1063/1.99947 Physics - 117 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Table (2): Properties of semiconductors ZnSe ZnS ZnO semiconductor Properties Zinc blend[10] Zinc blend[10] Wurtzite[10] Crystal structure o.566[10] 0.541 [10] a=0.32495,c=o. 5206[10 ] Lattice constant(nm) 5.42[10] 4.08 [10] 5.66 [10] Density(g/cm3) 9.2[10] 8.3 [10] 8.5 [10] Dielectric constant 2.62408[11] 2.52226[11] 2.00337[11 ] Refractive index 2.6[10] 3.6 [10] 3.4[10] Energy gab(eV) 4.09[13] 3.9[12] 4.5[12] Electron affinity(ev) Table (3): The results of calculation of the reoreintation energy λ(eV) for gold metal/semiconductor interface system Reorganization energy λ(eV) System d= as-m +1.6(A0) d= as-m +1.4(A0) d= as-m +1.2 (A0 ) d=as-m+1.0(A0) o.725354107 0.722411668 o.71900496 o.71502218 Au-ZnO 0.700700169 o.69202476 o.682074705 o.670556312 Au-ZnS o.705299904 o.696178713 O.685728201 o.673663776 Au-ZnSe Table(4):comparing our results with theoretical and experimental of reorientation energy λ(eV) Theoretical[ 14 ] Experimental[ 9 ] Our result 1.2-2.3 0.8 0.6-1.2 Au/ZnO 0.715 Au/ZnS 0.67 Au/ZnSe o.67 Physics - 118 ة و التطبيقيةمجلة إبن الهيثم للعلوم الصرف 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 معدن/حسابات النظرية لطاقة اعادة الترتيب عند سطح شبه موصل ال هادي جبار مجبل ، حسين خضيرمجبل قسم الفيزياء ، كلية التربية ابن الهيثم ،جامعة بغداد 2011ايلول 20قبل البحث: في 2011حزيران 19 :استلم البحث في الخالصة شبه -لالنتقال االلكتروني غير الكظيم على السطح لنظام مابين المعدنالحسابات النظرية لطاقة اعادة الترتيب الموصل استخرجت نظرية السطوح الكروية المستمرة لتفاعل االنتقال االلكتروني وصفت لالستعمال في االنتقال االلكتروني ما بين سطح المعدن وشبة الموصل. ابت العزل الكهربائي والبصري لشبه الموصل ، خواص المعدن ، وجد في الحسابات ان طاقة اعادة الترتيب تتناسب مع ث والمسافة الفاصلة بين شبه الموصل والمعدن. حيث يملك مالئمة جيدة Auاكثر مالئمة مع معدن الذهب ZnO اظهرت الحسابات لطاقة اعادة الترتيب أن شبه الموصل ا مع القيم العملية.. واظهرت الحسابات النظرية تطابقا جيد ZnSeو ZnSمقارنة مع ) ، Auشبه موصل، الذهب( /حسابات نظرية، طاقة اعادة الترتيب، المعدن، شبه الموصل، معدن -:الكلمات المفتاحية .ZnO ،ZnSe,ZnSاشباه الموصالت