Lecture: Week 15a
1. Carbon nanotube inverter
2. Simple and very useful nanotech tools: Scanning Tunneling
Microscope (STM) and Atomic Force Microscope (AFM)
2. How mechanical engineers use circuit analysis
3. What about radio?
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1. Single carbon nanotube inverter stage (from ???)
Figure 3 (a) AFM image showing the design of an intramolecular logic gate.
A single nanotube bundle is positioned over the gold electrodes to produce
two p-type CNTFETs in series. The device is covered by PMMA and a window
is opened by e-beam lithography to expose part of the nanotube. Potassium
is then evaporated through this window to produce an n-CNTFET, while the
other CNTFET remains p-type. (b) Characteristics of the resulting intramolecular
voltage inverter. Open red circles are raw data for five different measurements
on the same device (V = ±2 V). The blue line is the average of these five
measurements. The thin straight line corresponds to an output/input gain of one.
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2. The Scanning Tunneling Microscope (STM)
and the Atomic Force Microscope (AFM)
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2a. The Scanning Tunneling Microscope
The scanning tunneling microscope (STM) is a type of electron microscope that shows
three-dimensional images of a sample. In the STM, the structure of a surface is studied
using a stylus that scans the surface at a fixed distance from it.
Currents Control the Surface
An extremely fine conducting probe is held close to the sample. Electrons tunnel
between the surface and the stylus, producing an electrical signal. The stylus is
extremely sharp, the tip being formed by one single atom. It slowly scans across the
surface at a distance of only an atom's diameter. The stylus is raised and lowered in
order to keep the signal constant and maintain the distance. This enables it to follow
even the smallest details of the surface it is scanning. Recording the vertical movement
of the stylus makes it possible to study the structure of the surface atom by atom. A
profile of the surface is created, and from that a computer-generated contour map of
the surface is produced.
STM inventors Rohrer
and Binnig, IBM, Zurich
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http://en.wkipedia.org/wiki/Image:ScanningTunnelingMicroscope_schematic.png"
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Here scientists have positioned 48 iron atoms into a circular ring in order
to "corral" some surface state electrons and force them into "quantum"
states of the circular structure. The ripples in the ring of atoms are the
density distribution of a particular set of quantum states of the corral.
[Crommie, Lutz & Eigler, IBM Almaden Research Center]
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STM of a molecule a molecule assembled on copper from 8 cesium and 8 iodine atoms).
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2b. Example Micromechatronic System:
Atomic Force Microscope
http://stm2.nrl.navy.mil/how-afm/how-afm.html
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http://stm2.nrl.navy.mil/how-afm/how-afm.html
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Example Application: Atomic Force Microscopy/Data Storage: The Millipede
Target density: 1 terabit/sq. in.
http://www.zurich.ibm.com/st/storage/millipede.html
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Example Application: Atomic Force Microscopy/Data Storage
Target density: 1 terabit/sq. in.
thermal actuation/readout
http://www.zurich.ibm.com/st/storage/millipede.html
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2. How mechanical engineers can use electrical
circuit analysis
(Loosely excerpted from “Engineering Mechanics, vol. 2: Dynamics”, 4th Edition
by Meriam and Kraige, Wiley and Sons – a text sometimes used in ME 104
at UC Berkeley)
“Oscillating electrical circuits are characterized by differential equations of the
same type as those obtained for oscillating mechanical systems, such as a springmass-dashpot system. Their analysis is therefore similar to that of a mechanical
system. Any result obtained for the electrical circuit will also apply to the mechanical
system for which it is analogous.”
R
Support
Spring
x
Mass
If vertical force applied to mass
Is sinusoidal, as Pmsin (ωt), displacement
x is solution of
mx” + cx’ +kx = Pm sin (wt)
Damper
(dashpot)
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L
V msin(ωt)
Charge q is solution of
Lq” + Rq’ +(1/C)q = Vmsin(ωt)
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Characteristics of a mechanical system and its electrical analogue
Mechanical System
m mass
c coefficient of viscous damping
k spring constant
x displacement
v velocity
Pm applied force
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Electrical System
L inductance
R resistance
1/C reciprocal of capacitance
q charge
i current
Vm applied voltage
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3. What about radio?
a. In a communication system one often shifts the information content (e.g.,
a waveform from a microphone) to a different (usually higher) frequency for
transmission. Example: telephone carrying audible voice or music and
being transmitted via RF at hundreds of MHz or even optical beams (10141015 Hz.
b. Broadcast radio involves using the audio signals to modulate a steady
“carrier” signal at the transmitter; the received signal is then demodulated
at the receiver
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0
-10
dB -20
-30
-40
30
60
125 250 500 1k 2k 4k
Frequency of sound in Hz
(logarithmic scale)
8k 16k
Acuity of normal human hearing. Vertical axis: inverse of the
sound pressure level (SPL) corresponding to minimum detectable pure tone.
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Information transmission system – Purpose is to transfer
information from point A to point B accurately
Communication System
A
Source of
information
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Communication
channel
Transmitter
Receiver
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B
Recipient of
information
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Amplitude modulated (AM) radio communication system
Information
source
Tuner
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Pickup
(microphone
or
camera)
Amplitude
modulator
Amplifier
Antenna
Carrier
frequency
generator
Demodulator
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Amplifier
Output
(earphone,
loudspeaker,
display)
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Amplitude Modulation Radio (AM)
Voltage(V)
2
0
-2
0
1
2
Tim e (ms)
3
4
Sp ectral
density
Frequency in Hz
(expan ded scale
near carrier freLower fc a rrie r Up per quen cy)
Sideband
Sideband
Waveform (top) and spectrum (bottom) of AM radio station
broadcasting a 1000 H z tone. The spacing of the sidebands from the carrier
is determined by the frequency of the tone, in this case ±1000H z.
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Crystal radio receiver for AM broadcasts – no batteries!
+
L
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C1
C2
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Frequency modulated (FM) waveform
Frequency modulated carrier
Audio frequency waveform
to be transmitted
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PUZZLERS
1. Why does a nonlinear circuit generate “new” frequencies not present
in the driving signal?
2. How could you use a resonant circuit to “demodulate” an FM signal?
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