Lecturer: assist, prof. dr. Matjaž Kristl BjěHSEíMhjiB B3 University of Maribor, Sloveni; Faculty of Chemistry and Chemical Engineeri Department of Inorganic Chemistry Introduction: Fundamental terms about semiconductors energy bands, band gap i Doping, p- and n- type semiconductors i Semiconducting materials by groups MSI ill PRI iTt IITHMl > II - VI type ■ ' - > Cadmium chalcogenides, traditional and modern methods of preparation > Use of sonochemical method in the synthesis of CdS and CdSe Introduction ■ Semiconductor: material that has a resistivity value between that of a conductor and an insulator ■ The conductivity of a semiconductor material can be varied under an external electrical field ■ Devices made from semiconductor materials are the foundation of modern electronics: including radio, computers, telephones, and many other devices. ■ Semiconductor devices: transistor, diodes including the light-emitting diode (LED), integrated circuits ■ Solar photovoltaic panels: large semiconductor devices that directly convert light energy into electrical energy. Energy bands, band gaps Electrons of a single isolated atom occupy atomic If several atoms are brought together into a molecule, their atomic orbitals split - the number of moleculr orbitals proportional to the number of atoms When a large number of atoms are brought together to form a solid, the number of orbitals becomes exceedingly large, and the difference in energy between them becomes very small, so the levels may be considered to form continuous bands of energy rather than the discrete energy level ■ Some intervals of energy contain no orbitals, no matter how many atoms are aggregated, forming band gaps Semiconductors: E_ < 3 eV o Ü LU overlap Conduction band Fermi level Valence band Bandgap metal semiconductor insulator Doping Intrinsic (undoped) semiconductor: conductivity due to riEWlEHTŽTiE n - type doping: adding an impurity of valence 5 element to a valence 4 semiconductor, typically Si + P p - type doping: adding an impurity of valence 3 electron to a valence 4 ypically Scheme of basic princi U 4 neutral region p fin prd space charge i h (j k: n neutral region s: i n-doped ■Diffusion force" on holes E-NekJ force on holes "Diffusion force" on electrons E-fie Id force on electrons neutral region HI 1 neutral region AV Z<2 ^ - c y* Ol" t - t [ holes t: 0 © ftlpf trůilfi 1 e ©\ ®Á p-doped ee yá « n-doped e e/ $\$ ^^^^^^^^^^^_ 4e -^ +-— x E field Q1 i Charge 9 e a E\ Electric field v x V T Voltage r | AV built-in voltage J.__ t- ac Intrinsic (undoped) semiconductor: ■ n = p ■ Conductivity due to crystal defects or thermal excitations Doped semiconductors: ■ Classical Si cell: p - n (p - doped Si + n - doped Si) Recent designs: i p - i - n Si cells: the middle layer is intrinsic £I§I9||31NLtlt Common types of semiconducting materials Group IV elmental SC: Si, Ge Group IV compound SC: SiC, SiGe III - V semiconductors: GaAs II -VI semiconductors: CdS, CdSe, CdTe ZnO, ZnS, ZnSe, ZnTe ternary compounds, e. g. CdZnTe Cadmium sulfide, CdS Two naturaly occuring crystalline modifications Greenockit (hekxagonal UC): ■*ž* Hawleyit (cubic UC): Applicatons of CdS Known as cadmium yellow (CI pigment yellow 37): pigments valued for good thermal stability, light and weather fastness and high opacity Pigment in plastics and in art: Van Gogh, Monet Direct band gap semiconductor: band gap = 2.42 eV at 300 K (bulk), up to 4 eV with nanoparticles ■ > conductivity increases when irradiated —> I photoresistor I > Both polymorphs are piezoelectric I Solid state laser I > When combined with a p - type semiconductor: I photovoltaic (solar) cell: (CdS/Cu9S , 195^ Applications of CdSe Thermally stable pigment: CdS + CdSe = orange to red colours miconductinq material: band qap = 1.74 eV at tOO K Laser diodes Size dependent fluorescence spectrum (quantum confinement): properties of CdSe are tunable based on their size Tested for use in high - efficiency solar cells Cadmium telluride, CdTe Crystalline compound, zinc blende (cubic) crystal structure Direct band gap semiconductor: band gap = 1.56 eV at 300 K, strong solar cell material Highly usefull in making thin - film photovoltaic modules Alloyed with mercury: verstile IR detector material Alloyed with zinc: x-ray and gamma ray detector IR optical material for optical windows and lenses CdTe photovoltaics First and only photovoltaic technology to overtake silicon in cheapness Since the beginning: the dominant solar cell technology has been based on crystalline Si i Research in CdTe: late 1950s i Band gap: around 1.5 eV, perfect match to ion of ohotons in the solar spectrum ssr 1960s: simple heterojunctions, p-type CdTe + n -type Cd s pnHMBHHHBHHBIMI 44 40 h 28 h M ultijunction Concentrators Y Three-junction (2-terminal, monolithic) A Twojunction (2-ternninal, monolithic) Single-Junction GaAs A Single crystal A Concentrator VThinfilm Crystalline Si Cells ■ Single crystal □ Multicrystalline ♦ Thick Si film Thin-Film Technologies #Cu(ln,Ga)Se2 OCdTe 0 Amorphous Si:H (stabilized) ♦ Nm, micro-, poly-Si n Multijunction polycrystalline Emerging PV 0 Dye-sensitized cells house ♦ Organic cells (various technologies' Best Research-Cell Efficiencies Boeing-Spectrolab amorphic) X"-" Stanford (140xconc.) Varian (216xoonc, A Matsushita O i i i I Linz J__I__I__I__I__I__I__I__I__I__I__I__L 1995 2000 2005 1980 Main concerns connected with CdTe cells Te supply: > Recently 8001 / year > Coproduct with Cu productior > Few uses - few exploration (new places in China) Toxcicity of Cd: > CdTe is toxic, but only if ingested or inhaled > Securely encapsulated, can be rendered harmless > Recycling of modules at the end of their lifetime > More environmental friendly than any other use of Classical methods of synthesis Solid - state reactions Reaction between aqueous solutions of Cd - salts and gaseous H2S / H2Se ♦:♦ In the past: gravimetric analysis of cadmium: using gaseous, highly toxic reactant! Pyrolysis CdS thin films: from Cd - salts and thiourea or from volatile Cd - alkyls CdSe: preparation of bulk material by high pressure vertical zone melting Modern methods for the synthesis of cadmium chalcogenides Safer, avoiding toxic reactants Milder reaction condidtions —► easier to control ■■lllIWSllttMIIKllilSISI synthesis .n liquid ammonia (amorphous product!) Bacterial biosynthesis Mechanochemial method (mechanical alloying, high energy milling) Basic principles of sonochemical reactions Ultrasound: cyclic sound pressure with fequencies between 20 kHz and 10 MHz Medical and Destructive Animals and Chemistry Diagnostic ÍO0MHE Infrasound Acoustic Ultrasound Effect of ultrasound to molecules: indirect, most probable through the mechanism called acoustic cavitation: formation, growth and implosive collapse of gas/vapour bubbles inside the liquid Extreme conditions at the collapse1 ('hot - spot'): T > 5000°C, P > 2000 bar, AT / At ~ 109 K/s 11 1 Polmer mehurckov (um) 150 100 50 Kavitacijska jedra 1 100 200 300 Čas (ms) 400 500 600 Photograph of a collapsing bubble during acoustic cavitation, 20 million frames / s • • m m Used ultrasonisc system Sonics & Materials VCX 750 1.25 cm2 Ti - probe 20 KHz 100 W/cm2 50 m L beaker Conclusion: Simple sonochemical methods, suitable for preparation of semiconducting CdS and CdSe Nanocrystalline particles with average particle size 4,54 - 9,66 nm Confirmation by X - ray powder diffraction, Detailed thermal analysis in air and N2 flow Further investigations underway: attempt to prepare CdTe nanoparticles in similar way! Acknowledgment Ministry of Higher Education, Science and Technology of the Republic of Slovenia for the financial support Sašo Gyergyek, M. Sc, for TEM images an EDS spectra Anita Dane and Valerija Dane, graduate students, for laboratory work