TeerthankerMahaveerUniversity, Moradabad
College of Engineering
CT-2 (Even Semester) Examination 2015-16 For Ist Year/ IInd Semester (
M.Sc. Physics
) Sub. Name: Nanoscience& TechnologyMax Marks: 50Subject Code: MPH 204 Duration: 01:45 hr.
Course/Branch/Section:M.Sc./Physics/IstYr
(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) Write short notes on mesoscopic magnetism.
(b) Explain, how nanotechnology is useful for magnetic recording technology? (c) Write the uses of biological nanomagnets.
Section –B(Unit II)
(d) What do you understand by nanomachines? (e) Write the potential application of MEMS.
(f) What do you understand by the term nanolithography?
Q2(A):Briefly explain DC SQUID micro - susceptometry. Write its uses. or [15x1=15] (B):What do you understand by miniature Hall detector?
Q3(A):Briefly explain the dynamics and applications of NEMS.
or [15x1=15] (B):Briefly explain the working of a single electron transistor (SET).
1 (a):A nanomagnet is a sub micrometric system that presents spontaneous magnetic order (magnetization) at zero applied magnetic fields (remanence).The small size of nanomagnets prevents the formation of magnetic domains. The magnetization dynamics of sufficiently small nanomagnets at low temperatures, typically single-molecule magnets, presents quantum phenomena, such as macroscopic spin tunnelling. At larger temperatures, the magnetization undergoes random thermal fluctuations (super-paramagnetism) which present a limit for the use of nanomagnets for permanent information storage.
1 (b):Nano Storage: Miniaturized magnetic devices are omnipresent in the hard disks of computers and in tape storage systems. As a result of intensive research and development, storage density on hard disks has increased dramatically at a rate greater than 60% per year. At nano-scale dimensions the properties of magnetic devices are strongly affected by their size and shape in a complex way resulting from the interplay between different types of magnetic energy. Arrays of nanomagnets could be used for ultra-high density storage on hard disks or for fast and dense, non-volatile, solid state memory.The growth in the storage capacity of the ubiquitous hard disk drive (HDD) has been extraordinary and has been achieved by the miniaturisation of components (the read/write head geometry, media thickness, grain diameter) and the development of novel recording media.
1 (c):BIOLOGICAL MAGNETS:The name Nanobiomagnetismis the combination of magnetism, nanotechnology and life science. The Nano-scale size of these particles are comparable with the size of viruses, proteins, cells and genes, making it compatible for study of life processes at cellular level. The magnetic property not only helps us to control the movement of Nano magnets inside the body, localizing its position with the help of an external magnetic
field, but also enables heating of local tissues using sufficient strength and optimum frequency. The special properties of Nano magnets have resulted in its extensive use in medicine, revolutionizing the new approach of targeted treatment. Nano magnets can be effectively used for different medical applications like drug targeting, drug delivery and release, hyperthermia and as contrast agents in magnetic resonance imaging (MRI).
1 (d):Nanomechanics is a branch of nanoscience studying fundamental mechanical (elastic, thermal and kinetic) properties of physical systems at the nanometer scale. Often, nanomechanics is viewed as a branch of nanotechnology, i.e., an applied area with a focus on the mechanical properties ofengineered nanostructures and nanosystems (systems with nanoscale components of importance). Examples of the latter includenanoparticles, nanopowders, nanowires, nanorods, nanoribbons, nanotubes, including carbon nanotubes (CNT); nanoshells, nanomebranes, nanocoatings, nanocomposite/nanostructured materials, etc.
1 (e):The real potential of MEMS starts to become fulfilled when these miniaturized sensors, actuators, and structures can all be merged onto a common silicon substrate along with integrated circuits (i.e., microelectronics). While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. It is even more interesting if MEMS can be merged not only with microelectronics, but with other technologies such as photonics, nanotechnology, etc. This is sometimes called “heterogeneous integration.”
1 (f):Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scalestructures, meaning patterns with at least one lateral dimension between 1 and 100 nm. Different approaches can be categorized in serial or parallel, mask or maskless/direct-write, top-down or bottom-up, beam or tip-based, resist-based or resist-less methods. As of 2015, nanolithography is a very active area of research in academia and in industry. Applications of nanolithography include among others: Multigate devices such as Field effect transistors (FET), Quantum dots, Nanowires, Gratings, Zone plates andPhotomasks, nanoelectromechanical systems (NEMS), or semiconductor integrated circuits (nanocircuitry).
2(A): SQUID – MAGNETOMETER
A superconducting quantum interference device (SQUID) is a mechanism used to measure extremely weak signals, such as subtle changes in the human body's electromagnetic energy field. Using a device called a Josephson junction, a SQUID can detect a change of energy as much as 100 billion times weaker than the electromagnetic energy that moves a compass needle.
DC SQUIDs, contained in a helmet-like device, measure the currents created by neural activity. The possible SQUID neuroscience applications are myriad. A recent study used SQUID-enabled to measure the surprisingly large level of activity in consumer's brains.
DC SQUID
The current I enters and splits into the two paths, each with currents Iaand Ib. The thin barriers on each path are
Josephson junctions, which together separate the two superconducting regions. Φ represents the magnetic flux threading the DC SQUID loop. Ib is the bias current, I0 is the critical current of the SQUID, Φ is the flux threading
the SQUID and V is the voltage response to that flux. The X-symbols represent Josephson junctions.
The DC SQUID has two Josephson junctions in parallel in a superconducting loop. It is based on the DC Josephson effect. In the absence of any external magnetic field, the input current splits into the two branches equally. If a small external magnetic field is applied to the superconducting loop, a screening current, , begins circulating in the loop that generates a magnetic field canceling the applied external flux. The induced current is in the same direction as in one of the branches of the superconducting loop, and is opposite to in the other branch; the total current becomes in one branch and in the other.
Now suppose the external flux is further increased until it exceeds Φ , half the magnetic flux quantum. Since the flux enclosed by the superconducting loop must be an integer number of flux quanta, instead of screening the flux the SQUID now energetically prefers to increase it to Φ . The screening current now flows in the opposite direction. Thus the screening current changes direction every time the flux increases by half integer multiples of Φ . Thus the critical current oscillates as a function of the applied flux. If the input current is more than , then the SQUID always operates in the resistive mode. The voltage in this case is thus a function of the applied magnetic field and the period equal to Φ . Since the current-voltage characteristics of the DC SQUID is hysteretic, a shunt resistance, R is connected across the junction to eliminate the hysteresis (in the case of copper oxide based high-temperature superconductors the junction's own intrinsic resistance is usually sufficient). The screening current is the applied flux divided by the self-inductance of the ring. Thus Φcan be estimated as the function of (flux to voltage converter) as follows:
∆V = R ∆I
2I = 2 ∆Φ/L, where L is the self inductance of the superconducting ring ∆V = (R/L) ∆Φ
According to the relations, given above, this implies also small current and voltage variations. In practice the self-inductance L of the loop is not so large. The general case can be evaluated by introducing a parameter
Φ with ic the critical current of the SQUID. Usually λ is of order one.
2(B):HALL DETECTOR:Micro-Hall sensors are sensitive tools to examine magnetization patterns on a nanoscale. Micro-Hall-magnetometry together with complementary imaging techniques such as, Lorentz- and magnetic force microscopy provide important insights in the magnetic switching process of ‘mesoscopic’ magnets.
Miniaturizing the device dimensions is of interest because of the large area of magnetism. In this transition regime the magnetic properties of these so called nanomagnets can be tailored by varying their shape, size, material and structure. This makes them an interesting subject mainly in memory and data storage technology.
Principles of Hall-magnetometry
Micro-Hall-magnetometry, in contrast, imposes only a negligible perturbation on the nanomagnet during the magnetization reversal process. The magnetic field caused by the sensor current is only on the order of 10 µT. An important advantage of this method is that it can be employed over a wide range of temperatures.
enhancement of the voltage signal by orders of magnitude can be obtained by applying semiconductor Hall devices. Extremely sensitive Hall sensors can be fabricated from GaAs/AlGaAs semiconductor heterostructures providing a two dimensional electron gas (2DEG) only some ten nanometers below the surface. The mobility and the density of 2DEG electrons are typically several 105 cm2/V s and some 1011 cm−2, respectively.
One possible approach to apply the Hall sensors for probing stray fields on a sub-micron scale is Scanning Hall Probe Microscopy (SHPM). A magnetic tip, a micro-Hall sensor is scanned across the surface to probe the local magnetic stray field by measuring the resulting Hall voltage. The guiding of the micro-sensor is accomplished by the scanning unit of a scanning microscope the probe is attached to.The sample sensor distance can be adjusted e. g., by a shear force distance control. Recording the Hall voltage across the scanned area gives a complete map of the magnetic field distribution of a magnetic surface. The highest accessible lateral resolution of nanoscale Hall sensors based on 2DEG systems is estimated to be of order ≈ 50 nm.
The cantilever which is sandwiched between two piezo plates (gray) oscillates at its resonance frequency driven by one of the piezos. The amplitude is detected by the second piezo plate and serves as the control signal for the scanner z-piezo element to maintain constant sensor-sample distance. Instead of moving micro-Hall sensors as scanning probes they can be utilized as miniaturized magnetometers to study magnetization reversal of nanomagnets. The basic idea is to pattern the magnetic particle to be examined directly onto the Hall cross sensor. This technique, which is illustrated by the sketch in Fig. 4, has become a powerful method for the investigation of micron and sub-micron size magnetic particles, as will be demonstrated by several examples in the following sections.Hall sensors might also be used to investigate biological systems.
3 (A): Nanoelectromechanical systems: NEMS are nanometer scale sensors, actuators, devices and systems with critical feature sizes ranging from 100 to a few nanometers. NEMS form the logical next miniaturization step from MEMS devices. NEMS typically integrate transistor-like nanoelectronics with mechanical actuators, pumps, or motors, and may thereby form physical, biological, andchemical sensors. Different properties of the NEMS-based devices, which makes them unique are low mass, high electrical strength, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion and a high surface-to-volume ratio useful for surface-based sensing mechanisms. Uses include accelerometers, or detectors of chemical substances in the air.
Approaches to miniaturization: Two complementary approaches to fabrication of NEMS can be found. The top-down approach uses the traditional microfabrication methods, i.e. optical and electron beam lithography, to manufacture devices. Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components. This allows fabrication of much smaller structures, albeit often at the cost of limited control of the fabrication process.
A combination of these approaches may also be used, in which nanoscale molecules are integrated into a top-down framework. One such example is the carbon Nanotube nanomotor.
Carbon allotropes:
Many of the commonly used materials for NEMS technology have been carbon based. This is mainly because of the useful properties of carbon based materials which directly meet the needs of NEMS. The mechanical properties of
carbon (such as large Young's modulus) are fundamental to the stability of NEMS while the metallic and semiconductor conductivities of carbon based materials allow them to function as transistors.
Difficulties: CNT and graphene for NEMS technology, face several hindrances to their implementation. One of the main problems is carbon’s response to real life environments. Carbon nanotubes exhibit a large change in electronic properties when exposed to oxygen. Graphene also has very complicated electric conductivity properties compared to traditional semiconductors as it lacks an energy band gap . This means that traditional constructions of electronic devices will likely not work and completely new architectures must be designed for these new electronic devices.
Future of NEMS: Key hurdles currently preventing the commercial application of many NEMS devices include low-yields and high device quality variability. The next challenge to overcome involves understanding all of the properties of these carbon-based tools, and using the properties to make efficient and durable NEMS with low failure rates.
3(B): Single Electron Transistor: The single-electron transistor (SET) is a nanodevice that can control the transport of single elementary charges on and off a metallic island. It can also function as transistor similarly to a nowadays FET. The principles of the operation of the SET is determined by the Coulomb blockade, an energy barrier that determines the current flow through the device and the charge placed on the metallic island. Regulating the gate charge of the device can modify the Coulomb blockade. The SET can also be used as an ultra-sensitive electrometer in DC ad RF mode. Theoretical calculations show a charge sensitivity h values lower than 1.7106e/ Hz for the SET and experimental research gives values of 1.2105e/ Hz . The experimental value for the SET is 1000 times better than the field-effect transistor used as an electrometer. The SET can thus be used as ultra-sensitive electrometer and will be used in the future in the study of charged nanoscale systems.
Principles of the Single Electron Transistor The Coulomb Blockade
The single-electron transistor consists of a metallic island, placed between two tunneling junctions connected to a drain and a source and has a gate electrode as in a normal field-effect transistor. The tunneling junctions are simply a thin (<10 nm) oxide layer between the island and the electrodes. Quantum dots have also been used as islands for the SET. The schematics of the SET are given in figure 1. Each tunneling junction in the SET has intrinsic tunneling resistance and capacitance (parallel to each other).
The island of the single-electron transistor, even if very small (nanometric scale) still contains a very large number of electrons (109). Yet, through tunneling, one can add or subtract electrons from the island charging it either negatively or positively. The extra electrons that charge the island are called excess electrons and their number is designed by n. The number of excess electrons can also be negative, meaning that electrons have been removed leaving a positive charge on the island (one could talk of excess holes in this case). The presence of excess electrons affects the electrostatic energy of the system, which depends on the charging energy of the SET:
C e n C
Q
Ech isl
2 2 2
2 1 2
1
whereQisl is the charge on the island, n the number of excess electrons, e the charge of one electron and CS the total
capacitance of the island which is equal to: C CG CL CR (CG, CL and CR are the gate capacitance and the
intrinsic capacitances of the left and right tunneling junctions respectively). The energy scale applied when working
with the SET is usually defined on the charging energy itself and the unit taken is usually:
C e EC
2
2
. The energy
does not only depends on Qisl, but also on the charge induced by the gate, the gate charge QG=VGCG where VG is the
gate voltage. The electrostatic energy of the system is equal to Eel EC(nng)2, where n is the number of excess electrons of the island and ng the number of elementary gate charges. The expression for the electrostatic energy of
the system then becomes:
C Q ne C
C V ne C
Q
E G G G
el
2
2 ( )
2 1 2 ) (
2 1 2
1
(2)
This energy determines if tunneling through a junction is forbidden or allowed: if the adding of an extra excess electron causes the energy of the system to increase then tunneling will be energetically forbidden and the Coulomb charging energy will act as a blockade. This is known as the Coulomb blockade.
