TeerthankerMahaveerUniversity, Moradabad
College of Engineering
CT-1 (Even Semester) Examination 2015-16 For IIIrd Year/ VIth Semester (
B.Sc. Physics
) Sub. Name: Int. to Nanoscience&Tech.Max Marks: 50Subject Code: BSA608 Duration: 01:45 hr.
Course/Branch/Section:B.Sc./Physics/IIIrdYr
(First – 15 Min.are for distribution and reading of the paper & paper writing time 1 Hr 30 Min.)
Note: Attempt all questions. Question number 1 is compulsory.
Q1: Attempt any two parts from each section. [4×5=20]
Section –A(Unit I)
(a) What is nano scale system? Explain its significance in nano technology. (b) Define quantum confinement of nano particles.
(c) Write short notes on top down and bottom up approach.
Section –B(Unit II)
(d) Explain the significance of excitonic Bohr radius.
(e) What are quantum dots? How it can be differentiated with nano particles? (f) Write short notes on colloidal methods for the preparation of nano particles.
Q2(A):Find the solution of wave function for quantum confinement of nano-particles in 3D. or[15x1=15]
(B): Discuss Landauer-Buttiker formalism for conduction in confined geometries.
Q3(A):Describe briefly the epitaxial growth method of nanoparticle. or [15x1=15]
(B): Briefly explain the optical spectroscopy methodswith reference to quantum dots.
1. (a) Nanoscale materials are defined as a set of substances where at least one dimension is less than approximately 100 nanometers. A nanometer is one millionth of a millimeter - approximately 100,000 times smaller than the diameter of a human hair. Nanomaterials are of interest because at this scale unique optical, magnetic, electrical, and other properties emerge. These emergent properties have the potential for great impacts in electronics, medicine, and other fields.
(b) The quantum confinement effect is observed when the size of the particle is too small to be comparable to the wavelength of the electron.To understand this effect we break the words like quantum and confinement, the word confinement means to confine the motion of randomly moving electron to restrict its motion in specific energy levels (discreteness) and quantum reflects the atomic realm of particles.So as the size of a particle decrease till we a reach a nano scale the decrease in confining dimension makes the energy levels discrete and this increases or widens up the band gap and ultimately the band gap energy also increases.
(c) Two main techniques used are:
(i) Bottom up approach: In this technique materials and devices are made up atom by atom. It provides
components made of single molecules, which
components. Furthermore the amount of information that could be stored in devices build from the bottom up approach would be enourmous.
(ii) Top down approach: In this technique materials are disassemble (break, or dissociate) bulk solids into finer pieces until they are constituted of only a few atoms. This domain is a pure example of interdisciplinary work encompassing physics, chemistry, and engineering upto medicine.
(d) When sizes are comparable to or smaller than the Bohr radius, the dimensions of the nanoparticle itself defines the spatial extent of the electron - hole pair (exciton) state, and hence the size of the spatial confinement of electronic wave-functions, which is known as “quantum confinement”. As the nanoparticle size is reduced, the electronic excitations shift to higher energy, and the oscillator strength is concentrated within just a few transitions, making their electronic states become discrete.
Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. This size dependency is clearly remarkable in the emission.
Quantum confinement not only produces size-dependent band gap of nanocrystals, but also size-dependent extinction coeffcient, which indicates how much light is absorbed by the material at a certain wavelength, per mass unit or molar concentration.
(e)A quantum dot is a nanoparticle made of any semiconductor material such as silicon, cadmium selenide,cadmium su lfide, or indium arsenide. Nanoparticles is typically used for particles in the nm size regime, while quantum dots are those nanoparticles that are in "quantum size regime" characterized by the discretization of the energy levels inside the material. For semiconductor nanoparticles, the quantum size regime is obtained when their dimensions are smaller than the exciton Bohr radius (for example in CdS such a threshold value is about 5.4nm). For metal nanoparticles, is not so easy to define the conditions for the quantum size regime.
(f)Colloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The synthesis of colloidal quantum dots is done by using precursors, organic surfactant, and solvents. Heating the solution at high temperature, the precursors decompose forming monomers which then nucle
ate and generate nanocrystals. The temperature and concentration of monomers,
during the synthetic process are critical factors in determining optimalconditions for the nanocrystal growth. Typical dots are made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, andindium phosphide. Due to this scalability and the convenience of benchtop conditions, colloidal synthetic methods are promisi ng for commercial applications.
(B): Landauer-Büttiker formalism: In 1957, Rolf Landauer proposed that conduction in a 1D system could be viewed as a transmission problem. For the 1D GNRFET on the right
(where the graphenenanoribbon channel is assumed to be ballistic), the current from A to B (given by the Boltzmann transport equation) is
where due to spin degeneracy, e is the electron charge, h=Planck's constant, and are the Fermi levels of A and B,
M(E) is the number of propagating modes in the channel, f’(E) is the deviation from the equilibrium electron distribution (perturbation), and T(E) is the transmission probability (T=1 for ballistic). Based on the definition of conductanceG=I/Vand the voltage separation between the Fermi levels is approximately , it follows that where M is the number of modes in the transmission channel and spin is included. G is known as the quantized conductance. The contacts have a multiplicity of modes due to their larger size in comparison to the channel. Conversely, the quantum confinement in the 1D GNR channel constricts the number of modes to carrier degeneracy and restrictions from the material's energy dispersion relationship and Brillouin zone. For example, electrons in carbon nanotubes have two intervalley modes and two spin modes. Since the contacts and the GNR channel are connected by leads, the transmission probability is smaller at contacts A and B, . Thus the quantum conductance is approximately the same if measured at A and B or C and D.
Landauer-Buttiker formalism holds as long as the carriers are coherent (which means the length of the active channel is less than the phase-breaking mean free path) and the transmission functions can calculated from Schrödinger's equation or approximated by the WKB approximation. Therefore, even in the case of a perfect ballistic transport, there is a fundamental ballistic conductance which saturates the current of the device with a resistance of approximately (spin degeneracy included). There is, however, a generalization of the Landauer-Büttiker formalism of transport applicable to time-dependent problems in the presence of dissipation.
Q3 (A): Describe briefly the epitaxial growth method of nanoparticle.
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate.Theoverlayer is called an epitaxial film or epitaxial layer. Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed crystal, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal.
Methods :Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor deposition. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductors. Solid-phase epitaxy is used primarily for crystal-damage healing.
Vapor-phase: Silicon is most commonly deposited by doping with silicon tetrachloride and hydrogen at approximately 1200 °C:
SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g)
This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching) may occur if too much hydrogen chloride byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:
SiCl4(g) + Si(s) ↔ 2SiCl2(g)
Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:
SiH4 → Si + 2H2
This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide.
Liquid-phase: Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid substrates. This happens at temperatures well below the melting point of the deposited semiconductor. The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition, the deposition of the semiconductor crystal on the substrate is relatively fast and uniform. The most used substrate is indium phosphide (InP). Other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, and to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar.
Solid-phase: Solid Phase Epitaxy (SPE) is a transition between the amorphous and crystalline phases of a material. It is usually done by first depositing a film of amorphous material on a crystalline substrate. The substrate is then heated to crystallize the film. The single crystal substrate serves as a template for crystal growth. The annealing step used to recrystallize or heal silicon layers amorphized during ion implantation is also considered one type of Solid Phase Epitaxy. The Impurity segregation and redistribution at the growing crystal-amorphous layer interface during this process is used to incorporate low-solubility dopants in metals and Silicon.[4]
Molecular-beam epitaxy:In molecular beam epitaxy (MBE), a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum (10−8 Pa; practically free space) to the substrate, where they condense. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing periodic groupsIII, IV, and V semiconductor crystals.
(B):
Spectroscopy
1) Near-field optical spectroscopy
Near-field optical probing is a powerful technique to study the optical properties of semiconductor quantum nano-structures with spatial resolutions well beyond the diffraction limit of light. The high-spatial resolution can approach λ/40, where λ is the optical wavelength of the dot luminescence, makes the imaging and spectroscopy of highly specialized nanostructures such as quantum dots possible, even at a spatial density of the order of 100/µm2.
The QD sample on the scanning stage is illuminated with He-Ne laser light through the aperture of a high sensitive double-tapered fiber probe under shear force feedback control. Most of the carriers are generated in GaAs and AlGaAs barrier layers. After diffusing in the barrier layers, the carriers are captured in the confined states of the QDs. The resultant PL signal from a single QD is collected by the same aperture to prevent the reduction of spatial resolution due to carrier diffusion. The PL signal is detected by means of a photon-counting avalanche photodiode through a bandpass filter. In the case of collecting the PL spectra, the PL signal is sent into a monochromator and detected by a cooled charged coupled device. The spectral resolution of this measurement is about 1meV.
To image a single dot, the luminescence falling inside a single window of Δλ = 2nm centered at 733nm is collected, and the tip is scanned across a 2μmX2μm area of the sample. The strong point of nano-probing is its ability to map the location, polarization state and emission wavelength of every dot even if the areal density is as high as 100/μm2
.
2) Far field Microscopy
Far-field epifluorescence confocal microscopy is another technique used to obtain images as well as spectra of single quantum dots. Excitation light is transmitted through a high reflecting mirror (at an angle of 45o) and focused by a long working distance microscope to a ~30 μm spot on the sample surface. The fluorescent image is collected by the same objective lens, reflected off the high reflecting mirror and passed through a wavelength specific filter to remove any excitation light while allowing all of the fluorescence to pass. The image is then focused onto the entrance slit of a spectrometer and detected with a cooled CCD camera (liquid nitrogen or thermoelectrically cooled). Both images and spectra of the sample can be collected on the same detector by switching between the diffraction grating and a mirror at 0o for spectra and images, respectively.
In order to take spectra from single nanocrystals, the entrance slit of the spectrometer is used to spatially isolate individual nanocrystals along a vertical region of the image. It is usually possible to align several nanocrystals at different vertical positions within the narrowed slit. The mirror in the spectrometer is then replaced with a diffraction grating and light from each vertical position is dispersed onto the CCD.
