Some examples of recent hot topics in Some examples of recent hot topics in Solid State Materials Solid State Materials 1)CNT & Graphene 2) Quantum dots & Energy harvesting materials 3) Conjugate polymers

Choi et al. APL 75, 3129 (1999) CNTs I-1. Typical applications of CNTs Hydrogen storage using SWNT

 Nanotube Transistor using Carbon Nanotube Bechtold et. al. Science 294 (2001) 1317 Height image of a single-nanotube transistor, acquired with an AFM. Schematic side view of the device. A semiconducting nanotube is contacted by two Au electrodes. An Al wire, covered by a few-nanometers-thick oxide layer, is used as a gate. Device characteristics of the nanotube transistor. In the linear regime at small source-drain voltages, the current is proportional to Vsd. In the saturation regime at higher source-drain voltages, the current through the transistor changes more gradually. For a constant Vsd in the saturation regime, I(Vg) has an approximately parabolic dependence I ~ (Vg - Vt) 2, as is typical for MOSFETs. Lines are guides to the eye. Large gain (>10) is obtaine

I-2. Typical applications of Graphene The electrons in graphene behaves as if they have no mass. Like photons—but unlike electrons in other materials—the electrons move at a constant speed, regardless of how much energy each one has. A transistor bulit out of graphene could operate much faster than a comparable one made from silicon.  Graphene Field-Effect Device

 Transparent Graphene Electrodes The self-assembly of soluble graphene sheets demonstrates the possibility of low-cost synthesis and fabrication of large-scale transparent films. K. S. Kim, Nature, 457 (2009) 457

What makes these excellent properties of CNTs and Graphene possible? One can start to investigate 1) their structural and electronic properties 2) also their lattice vibrational characteristics These two aspects can be researched by either experiment or theory.

II. Quantum dots  Particle in a box λ n = 2L/n (n=integer) Core : 1 ~ 10 nm size semiconductor inorganic spheres, rods, etc… Shells : Inorganic shell (complementary and/or protective) Surface group : passivating, protective and chemically active layer  Basic architecture of colloidal QDs  Quantum Dot (QD) Semiconductor nanoparticle that exhibits quantum confinement effect (typically less than 10nm in diameter) Consider a particle of mass m confined in a potential well of length L

 Emission of CdSe quantum dots Different particle size → different emission color Result of KAIST DML Quantum dots C.B. V.B C.B. V.B C.B. V.B

T. Tsutusi, Nature 420 (2002) 752 II-1. Application of QDs  Electroluminescence of QD-LEDs An applied electric field causes electrons and holes to move into the nanocrystal layer, where they are captured in the quantum dots and recombine, emitting photons.

 Bright, multicolored QD-LEDs Nature Photonics 1, (2007) High performance red, orange, yellow and green QD-LEDs based on QDs with a CdSe core and a ZnS or CdS/ZnS shell

Donglu Shi et al., Adv. Mater. 19, (2007) Marker : CdSe/ZnS Quantum dot Drug deliver: Multiwall Carbon nanotube Biomarking materials for cancer therapy require : - intense emissions - appropriate storage geometries Quantum dot conjugated carbon nanotubes designed to satisfy both requirements  QD-activated luminescent carbon nanotubes for in vivo imaging

II-2. Energy harvesting materials When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the p-type layer. The pn-junction, a one-way road, only allows the electrons to move in one direction. A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect.solar energyelectricityphotovoltaic effect

 QD Sensitized Solar Cell in Recent Years Easy color tuning in wavelength over visible range. Tzarara Lo´pez-Luke et al, J. Phys. Chem. C 2008, 112, → PCE of 9.25% was achieved ! CdSe quantum dots help TiO 2 to gather the photo induced carriers TEM image of CdSe quantum dots showing an average particle size 3.5 nm.  Quantum-dot Sensitized Solar Cell Boosting the power conversion efficiency (PCE) of solar cell device through multiple carrier generation with a single photon.

Conventional Polymer Polymer (insulator) (insulator) Sakuji Ideka ’s laboratory “Synthetic Metals” “Synthetic Metals” By nonmetallic substances Possibility presentation as New Generation Material Conducting polymer Novel Prize for Chemistry 2000 Hideki Shirakawa Alan G. MacDiarmid Alan J. Heeger H.J.byun in Synthesis by Ш. Conjugated polymer : functionalized Organic material Conjugated organic materials also have Energy band gap just like inorganic semiconductor

A common attribute possessed by semiconducting Polymers is that they contain a conjugated carbon "backbone“. This is a series of alternate single and double carbon bonds. “Delocalization” “Conduction Band (LUMO)” Lowest Unoccupied Molecular Orbital LUMO HOMO Highest Occupied Molecular Orbital “Valence Band (HOMO)” Ш. Conjugated polymer : functionalized Organic material

Organic lighting emitting diode Organic solar cell Organic thin film transistor Cheap, Simple process, and Flexible