Handbook of Thermal Analysis and Calorimetry
Vol 5: Recent Advances, Techniques and Applications
M. E. Brown and P. K. Gallagher, editors
2008 Elsevier B.V.
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Chapter 3
MICRO-THERMAL ANALYSIS AND RELATED TECHNIQUES
Duncan M. Price
Exhaust Management Systems, BOC Edwards, Kenn Business Park, Kenn Road,
Clevedon, North Somerset, BS21 6TH, UK
Email: [email protected]
1. INTRODUCTION
The major difficulty with conventional thermal analysis techniques is that they
measure the response of the whole sample. If one observes a broad change in
behaviour on heating a specimen, this could be the result of a genuine effect in a
homogeneous system or be due to a series of overlapping responses from a
heterogeneous system, where there may be a gradation in properties throughout
the sample. Alternatively, a weak effect seen in the entire sample could arise
from a strong response from a minority component (e.g. an impurity) within in
the bulk of the material. The same statements are true of other classical
analytical techniques which require moderately sized amounts of material for
sampling.
It is not surprising, therefore, that many investigators have combined studies by
thermal analysis with such techniques as optical [1] or electron microscopy. For
example, Price and Bashir [2] studied the thermal behaviour of
poly(acrylonitrile) which had been compression moulded in the presence of
water. Under conditions of high pressure, water acts as a solvent for this
otherwise intractable polymer (which normally degrades on melting) allowing
the material to flow. On cooling, the specimen has an open structure with the
water trapped in pores within the polymer mass. Differential scanning
calorimetry (DSC) was used to study the formation of ice within the pores on
cooling. Analysis of the shape of the DSC curves show that the pore size
followed an approximately bimodal distribution, whereas scanning electron
microscopy indicated that a third category of larger pores existed which could
not be detected due to the fact that they were incompletely filled with water [2].
Thermomicroscopy is the direct observation of a sample (usually) by optical
microscopy (often with polarised light) as a function of temperature [1].
Frequently called “hot-stage (optical) microscopy”, this is a relatively common
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technique which is used widely for the study of historical artefacts [3],
polymorphic transitions in drugs [4], pyrotechnics [5], and also for following the
crystallisation of polymers [6,7]. Other forms of high-resolution imaging such
as electron and scanning probe microscopies have been combined with the use
of a variable temperature sample stage. Of particular relevance to this
discussion is the use of atomic force microscopy at temperatures other than
ambient [8,9]. Studies by Grandy et al. [10] illustrate the potential of
mechanical property imaging above and below transition temperatures of two
phase polymer systems. Particularly novel is the application of a method of
image analysis which seeks to assign a statistical probability of each pixel
comprising the image to be one phase or the other (or an interphase). This is
based upon the assumption that the contrast can be described using overlapping
distributions of signal intensity arising from each component. Fasolka et al. [11]
have published similar work using resonant frequency (rather than forced
oscillation) non-contact atomic force microscopy where they also obtain images
at a temperature between the glass-rubber transitions of a diblock polymer
system.
The major disadvantage of any method of thermomicroscopy (whether optical,
electron or scanning probe technique) is that the whole sample is heated at the
same time. Changes in the sample which occur outside the field of view are not
observed and if these changes are irreversible (e.g. the sample crystallises,
degrades, two phases mix or separate) then a new sample must be prepared.
Ideally, one would desire a means to obtain a high-resolution image of a
specimen under ambient conditions and then carry out thermal analysis of a
specific region, phase or contaminant identified using the image as a guide.
Reading [12], in a patent application filed in May 1992, proposed an instrument
based upon a scanning thermal microscope modified to perform spatially
resolved modulated(-temperature) differential thermal analysis. Although such
an apparatus was never constructed, subsequent development in a collaboration
with Pollock and Hammiche resulted in a working instrument and three more
patents [13-15]. Development and prototyping of this concept took place
through collaboration between Loughborough and Lancaster Universities in the
UK in conjunction with commercial and UK Research Council support. TA
Instruments Inc. (Newcastle, Delaware, USA) launched this equipment as a
commercial product in 1998. The instrument received the Gold Award at the
1998 PittCon analytical instrumentation conference and the R&D 100 award for
innovation. In addition, the UK government awarded the instrument “Millenium
Product” status.
Before considering the development of what has become known as “micro-
thermal analysis”, it is appropriate to review the history, technology and
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applications of scanning thermal microscopy so as to place this family of
techniques in context.
2. SCANNING THERMAL MICROSCOPY (STHM)
The inventions of the scanning tunnelling microscope (STM) [16] and the
atomic force microscope (AFM) [17] have allowed sub-micrometre and, at
times, atomic-scale spatially-resolved imaging of surfaces. Spatially-resolved
temperature measurements using optical systems are diffraction limited by the
wavelength of the radiation involved, which is about 5-10 µm for infrared
thermography and about 0.5 µm for visible light [18]. The spatial resolution of
near-field techniques (such as AFM) is only limited by the active area of the
sensor (which in the case of STM may be only a few atoms at the end of a metal
wire).
The first experiments in scanning thermal microscopy (SThM) were carried out
by Williams and Wickramasinghe [20] who employed a heated thin-film
thermocouple fabricated from a conventional STM tip. As the tip approached a
surface, it was cooled due to tip-substrate heat transfer. By using the
temperature sensed by the thermocouple as a feedback to maintain a constant
tip-substrate gap, this scanning thermal profiler could overcome the limitations
of STM and be used to image electrically insulating surfaces with a lateral
resolution of 100 nm. Since the feedback signal was based on maintaining a
constant probe temperature, this device could not be used to obtain true thermal
images of surfaces. Instead, the system measured a convolution of topography of
the specimen and its thermal conductivity in a non-contact mode by means of
heat leak from the thermocouple to the sample through a small air gap.
In an attempt to overcome the limitations of this method of SThM, Majumdar
[21] described the use of an AFM cantilever fashioned from a pair of dissimilar
metal wires (chromel and alumel) which met to form a thermocouple junction at
the tip. In this way, the conventional AFM force feedback mechanism could be
used to measure surface topography whilst at the same time mapping the
temperature distribution of energized electronic devices with sub-micrometre
resolution. Since this demonstration, a number of different probe designs have
been developed and progress has been made towards the measurement of
absolute thermal conductivities and 3-dimensional tomographic imaging. A
general discussion of SThM can be found in the reviews by Gmelin [22] and
Majumdar [18,19].
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2.1. Instrumentation for SThM
The atomic force microscope forms the basis of both scanning thermal
microscopy and instruments for performing localised thermal analysis. A
schematic diagram of an AFM is shown in Figure 1 [23]. The instrument
consists of a sharp tip mounted on the end of a cantilever which is scanned
across the specimen by a pair of piezoelectric elements aligned in the x-y plane.
As the height of the sample changes, the deflection of the tip in contact with the
surface is monitored by an optical lever arrangement formed by reflecting a laser
beam from the back of the cantilever into a photodetector [24]. The tip is then
moved up and down by a feedback loop connected to a z-axis piezo which
provides the height of the sample at each x,y position. Besides the topographic
information provided by rastering the tip across the sample, other properties can
be obtained by measuring the twisting of the cantilever as it is moved across the
sample (lateral force microscopy) [25]. This provides image contrast based on
the frictional forces generated from the sample-tip interaction. Other imaging
modes, such as force modulation and pulsed force modes can indicate the
stiffness of the sample [26]. An advantage of the AFM over the scanning
electron microscope is that little sample preparation is required because the
sample is not exposed to a high vacuum and electrically insulating materials can
be examined.
Figure 1. Atomic force microscope (schematic) [23]. (With permission from
Elsevier.)
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2.2. Probe design
Three methods have been used to combine the conventional AFM cantilever
with a means of localised thermometry.
2.2.1 Thermocouple cantilevers
The use of AFM with a thermoelectric element at the tip has been described
above. In an effort to improve the performance of a bare thermocouple tip,
Majumdar et al. [27] cemented a diamond shard to the junction so as to give a
harder tip with improved spatial resolution and reduced thermal resistance. The
same group also describe depositing successive layers of different metals so as
to make thermocouple pair on top of a standard “A-frame” AFM cantilever [28].
Fish et al. [29] borrowed technology from near-field scanning optical
microscopy to make a thermocouple derived from gold-coated glass
micropipettes containing a platinum core. Workers at Glasgow University
[30,31] have fabricated thermocouple probes using electron beam lithography
and silicon micromachining in order to deposit one or more thermocouple
junctions at the AFM tip. Such work leads to the possibility of building
thermopile sensors (with the incorporation of a heater) analogous to a miniature
heat flux calorimeter [32,33].
2.2.2 Resistance thermometry
Figure 2. Schematic diagrams of resistive SThM probes: a) Wollaston wire
type [34,35], b) micro-machined coated Si cantilever [37] and c) “data storage”
doped Si probe [55].
In 1994, Dinwiddie and Pylkki [34,35] described the first combined
SThM/AFM probes that employed resistance thermometry to measure thermal
properties. These were fashioned from Wollaston process wire. This consists of
a thin platinum/5% rhodium core (about 5 µm in diameter) surrounded by a
thick (about 35 µm) silver sheath. The total diameter of the wire is thus about
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75 µm. A length of wire is formed into a “V” and the silver is etched away at
the apex to reveal a small loop of Pt/Rh which acts as a miniature resistance
thermometer (Figure 2(a)). A bead of epoxy resin is added near the tip to act
serve as mechanical reinforcement and sliver of silicon wafer is glued to the top
of the probe to act as a target mirror for the laser employed in the cantilever
deflection detection mechanism [36]. This can be operated in two modes: a) as a
passive thermo-sensing element (by measuring its temperature using a small
excitation current) or b) as an active heat flux meter. In the latter case, a larger
current (sufficient to raise the temperature of the probe above that of the surface)
is passed through the wire. The power required to maintain a constant
temperature gradient between the tip and sample is monitored by means of an
electrical bridge circuit. In essence, this is equivalent to a power compensation
calorimeter. Mills et al. [37] describe similar probes in which the resistance
element is deposited across the apex of a silicon nitride pyramid similar to a
conventional AFM cantilever (Figure 2(b)).
In a passive mode, such devices function like thermocouple probes described
above. These can be used (for example) to map the temperature distribution in
energised electronic devices simultaneously with their topography [38,39]. If
the surface is illuminated with infrared radiation, the photo-thermal effect
arising from the absorption of energy specific to the infra-red (IR) active modes
of the specimen may be used to obtain the sample’s IR spectrum [40-47]. In the
active mode, the heat flow from the tip can be used to detect surface and sub-
surface defects of different thermal conductivity than the matrix [48,49].
Other resistive probe designs have been reported. For example, Li and
Gianchandani [50] have fabricated SThM probes using polyimide rather than
silicon as a substrate in a process similar to that of Mills et al. [37] except that it
exploits the mechanical flexibility of the polyimide to implement an assembly
technique that eliminates the need for probe removal or wafer dissolution. Use
of polyimide rather than silicon as a substrate offers a greater degree of thermal
isolation since its thermal conductivity is three orders of magnitude lower than
silicon [51]. These probes have been manufactured in a differential arrangement
with one probe being used as a reference. Edinger et al. [52,53] describe a
sensor consisting of a nanometre-sized filament formed at the end of a
piezoresistive atomic force microscope type cantilever. The freestanding
filament is deposited by focussed electron beam deposition using
methylcyclopentadienyl trimethyl platinum as a precursor gas. The authors
claim a spatial resolution of < 20 nm and, due to its small thermal mass, a high
sensitivity and fast response time. Leinhos et al. [54] have fabricated thermal
elements from silicon with a Schottky diode integrated into the probe tip. One
design of such a probe may also be used for simultaneous scanning near-field
optical microscopy as well as SThM and AFM.
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Chui et al. [55] also describe high resolution silicon (piezo)resistive cantilevers
designed not for SThM, but for high-density data storage. An array of probes
are used to “write” 10-50 nm diameter pits in a spinning polymer coated silicon
disc [56]. Read-back is achieved by making use of the increased cooling of the
tip when it encounters the indentations when the probes are operated in SThM
mode. Such an approach is essentially the reverse of micro-thermal analysis in
that the marks produced as a consequence of localised thermomechanical
experiments are detected by the effect of their topography on the heat flux rate
between the probe and the sample [57]. Schematic diagrams of the Wollaston,
micromachined silicon and “data storage” probes are shown in Figure 2(a-c).
The spatial resolution of these probes is of the order of 2, 0.2 and 0.02 µm
respectively. All of these designs are now commercially available.
2.2.3 Bimetallic sensors
Nakabeppu et al. [58] describe the use of composite cantilevers made from tin
or gold deposited on conventional silicon nitride AFM probes to detect spatial
variations in temperature across an indium/tin oxide heater. Differential thermal
expansion of the bimetallic elements causes the beam to bend. This movement
is monitored using the AFM optical lever deflection detection system. In order
to separate thermal deflection of the beam from displacement of the cantilever
caused by the sample topography, an intermittent contact mode of operation is
employed. Measurements were made under vacuum so as to minimize heat loss.
A more practical use of this technology is in the form of miniature chemical and
thermal sensors [59]. This approach has been used to perform thermal analysis
on picolitre volumes of material deposited on the end of a bimetallic cantilever
[60]. Arrays of such devices have applications as highly sensitive electronic
“noses”.
2.3 Quantitative SThM
Small-scale measurements of thermal conductivity and thermal diffusivity
would benefit the semiconductor and other industries where thermal transport
properties are significantly different to, and cannot be inferred from,
measurements at higher scales. Examples of key areas of modern technology
and science which might be expected to benefit include microelectronics,
cellular biology, forensic, pharmaceutical and polymer science etc. In theory,
heated thermal probes are capable of measuring the absolute thermal
conductivity of materials by the heat flux between the tip and the surface. In
practice, heat losses also occur both within the probe and to the atmosphere.
Furthermore, the contact area between the tip and the specimen is usually
unknown. Ruiz et al. [61] developed a simple method for converting heat flux to
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thermal conductivity by using hard materials of known thermal conductivity to
calibrate the system. This procedure was used to determine the thermal
conductivity of diamond-like nanocomposites to a precision of ±15%.
Gorbunov et al. [62-64] measured the change in heat flux as the probe
approached the sample surface, or was ramped in temperature in contact with the
specimen, so as to derive its thermal conductivity – again by calibration with
samples of known response. This approach has been used to study IR receptors
in snakes with a view towards designing artificial sensors which mimic those
found in nature [65]. Fiege et al. [66] used AC heating of the tip to measure the
thermal conductivity of silver and diamond, using gold as a single-point
reference material in order to estimate the contact area of the tip. Blanco et al.
[67] have used SThM to investigate qualitative differences in thermal
conductivity of carbon-carbon composites arising from different processing
histories.
Figure 3. Illustration of the convolution of surface roughness on the apparent
thermal conductivity (k) measured by the tip during a line scan. Darker
coloured material (left) has lower bulk thermal conductivity than lighter
coloured material. See text for additional explanation.
The thermal conductivity contrast image obtained by scanning thermal
microscopy represents a convolution of the true thermal transport properties of
the specimen with artefacts arising from changing tip-sample thermal contact
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area caused by any surface roughness of the specimen [48]. When the probe
encounters a depression on the surface, the area of contact between the tip and
sample increases, resulting in increased heat flux from the tip to the sample.
More power is required to maintain the tip temperature at the set-point value and
this is recorded on the image as an apparent increase in the local thermal
conductivity. The opposite is true when the probe meets an asperity. Visual
comparison of the thermal image with the topographic image (recorded
simultaneously) often shows that “features” in the former are correlated with
changes in height of the specimen – particularly at edges of such features where
the surface relief changes rapidly. Even with careful sample preparation (e.g.
cutting or polishing) it is almost impossible to avoid artefacts in the thermal
image not attributable to topography. In cases where there is little inherent
thermal contrast between parts of a heterogeneous specimen, interpretation of
the thermal image can be problematic. An example of this is illustrated in
Figure 3 for three cases: 1 – a homogenous material flat surface; 2 – the same
material with a depression and an asperity; 3 – a smooth interface between low
thermal conductivity and high thermal conductivity phases & 4 as 3, but with
added surface roughness.
In theory, it would be possible to construct an analytical model of the probe-
surface interaction based upon the local geometry of the probe tip and surface
[62-64]. Rather than develop a general model for heat transfer, an alternative
approach can be developed using a neural net algorithm [68]. Each digitised
image consists of a grid of points (pixels) which define the height of the sample
and the heat flux from the probe to the sample. For each pixel the local
topography is characterised by subtracting its height from the heights of the
surrounding points. This defines whether the probe rests in a depression,
asperity, flat surface, etc. The training set for the neural network comprises
images acquired on materials of homogeneous thermal properties with a range of
surfaces roughnesses. From these samples a table of topographic parameters
correlated with the thermal measurement is obtained for a wide range of surface
roughness. The ‘true’ thermal measurement is the value obtained on a perfectly
smooth surface. In practice, an average value from the smoothest available
surface can be taken as the ‘true’ value. In each case, the required value is
given as the measurement on a smooth surface.
The usual procedures for training, testing and validation of a neural network
can be applied to result in a method to take a pair of images (thermal and
topographic) and effectively “subtract” artefacts arising from surface roughness.
As an alternative to lengthy training with a variety of specimens, it is feasible to
use the images of a test specimen itself to train the network. This method relies
upon the sample having discrete areas of homogenous thermal conductivity to
provide internal calibration values for network training. Figure 4 shows the
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results of carrying out this type of image processing on a sample of polyolefin
packaging film. Despite careful sample preparation, differences in mechanical
properties result in a non-flat sample after cutting with a microtome. This
generates false contrast in the raw thermal conductivity image which is largely
eradicated by processing with a neural net which was trained using a series of
polymers of differing surface roughnesses.
Figure 4. Left – topography of microtomed cross-section through a multi-
layer polyolefin packaging film. Centre – raw thermal conductivity contrast
image. Right – thermal conductivity image post-processing with trained neural
net program.
Other methods of image processing have been devised which apply a statistical
analysis of pixel intensity distribution to enhance image contrast. Royall et al.
[69] have used such a method to discriminate between the substrate and coating
of a pharmaceutical compact whereby each pixel is defined to be one or the
other component according to the heat flux from the tip to the sample. This
simple “on/off” data treatment can be extended to assign a probability (displayed
as a grey-scale level) of being a certain component based upon the position of
the pixel’s intensity within the total distribution of values. This procedure is
illustrated using a non-prescription analgesic tablet containing paracetamol
(acetaminophen) described below.
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Figure 5. (a) Raw thermal conductivity contrast image of an analgesic tablet
containing paracetamol [70], (b) histogram showing distribution of
pixel intensity fitted to two overlapping Gaussian peaks assigned to
the drug and filler components.
Figure 5(a) shows the raw thermal conductivity image of an analgesic tablet
containing the drug paracetamol (acetaminophen). Localised thermal analysis of
the sample indicates that the bright area in the bottom right of the image is some
form of thermally inert filler whereas the dark regions comprise the drug [70].
Inspection of a histogram of pixel distribution in Figure 5(b) shows that the
shape of this distribution can be modelled using two overlapping Gaussian peaks
corresponding to the drug and filler responses.
Figure 6. (a) Simple black and white separation of image shown in Figure
5(a) using a threshold of 1.66 mW, (b) Grey-scale separation of image shown in
Figure 5(a) using a more complete statistical analysis of the original histogram
distribution in Figure 5(b).
A simple means of highlighting the two phases might be to assign a zero grey-
scale intensity to pixels with an original value below the cross-over between the
two peaks at 1.66 mW and a grey-scale intensity of unity to all the pixels above
this threshold. Thus, points which are statistically more likely to be comprised
of the drug appear black and those which are more likely to be the filler appear
white (Figure 6(a)). This is the same, simple method of image processing that
has also been found to be particularly applicable to mechanical property imaging
modes (pulsed force mode atomic force microscopy) in combination with a
variable temperature sample stage [10]. A more refined approach is to generate
a grey-scale value for each pixel based upon the probability of it belonging to
the drug or filler component derived from the ratio of the value of the fitted
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peak height assigned to the filler to the total fitted histogram intensity. The
resulting transformation of the thermal image in Figure 5(a) via this process is
shown in Figure 6(b) and exhibits a more satisfactory discrimination of phases –
particularly at the interface between the two domains where there exists some
uncertainty over their assignment.
De Cupere and Rouxhet [71] have recently published a similar method of
image contrast enhancement based upon splitting a surface friction image into
two components using the pixel intensity histogram. The resulting image was
cleaned by selectively erasing any foreground pixel in contact with a
background pixel (“erosion”) and the final image was obtained by reconstruction
of the objects left after erosion. This procedure, which is similar to that
described above, was used to measure the surface fraction of spherulites grown
on amorphous poly(ethylene terephthalate) film after different annealing times.
2.3 Other SThM techniques
2.3.1 3-D tomographic imaging
The decay length of thermal waves produced by AC heating of a tip varies as a
function of the reciprocal of its frequency [72]. Thus, it is possible to detect
variations in thermal response at shallower depths by using a high frequency
temperature modulation superimposed on the conventional DC heating of the
tip. Several groups have employed this technique to study the thermal
diffusivity variations in materials [49,73,74]. Gomès et al. have examined this
process theoretically [75] and there is potential for the use of multiple frequency
modulated-temperature SThM as a means to provide non-destructive three-
dimensional imaging of optically opaque samples using similar principles to
those employed for medical imaging by electrical impedance tomography [76-
79].
2.3.2 Thermal expansivity imaging
An additional imaging mode has been demonstrated whereby the thermal
expansion of a specimen is detected whilst AC heating is applied to the probe as
it is scanned over the surface [23,70,80,81]. The resulting z-axis modulation of
the probe arising from thermal expansion and contraction of the surface is
detected and used to construct an image based on the thermal expansivity of the
sample. Although Majumdar [82-84] has also described similar measurements,
this approach does not require electrically conductive specimens and employs
the same configuration used for thermal conductivity imaging. Again, there is
the potential for using this approach for tomographic imaging.
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3. LOCALISED THERMAL ANALYSIS
3.1 Principles
Active, resistively-heated, thermal probes readily lend themselves to localised
thermal analysis. Although it is feasible to control the temperature of a
thermocouple-based probe by passing a current through it [86], the simultaneous
measurement of its temperature is non-trivial and requires filtering out of the
current providing the heating. Probes based on resistance thermometers are
more readily controlled and this technology forms the basis of the well-known
power-compensation differential scanning calorimeter [87]. Using a previously
acquired topographic and/or thermal image obtained by the SThM facility, it is
possible to place the probe at one or more selected locations in sequence on the
sample and program the tip’s temperature in order to make localised
measurements of transition temperatures. By monitoring the power required to
follow the temperature programme, a form of spatially resolved calorimetry may
be carried out [49,89]. In addition, since the vertical deflection of the tip can be
determined using the AFM stage, localized thermomechanical analysis (TMA)
may be performed concurrent with the calorimetry [72,90]. A linear temperature
ramp of the order of tens of degrees Celsius per second is the most commonly
employed programme, often with a superimposed sinusoidal modulation of the
probe temperature at kilohertz frequencies about the mean set-point. The use of
AC heating allows two extra calorimetric signals to be obtained – the AC power
and phase difference between the applied modulation and probe response – akin
to AC calorimetry [88]. Such high heating (and cooling rates) are possible as a
consequence of the small size of the probe and the region (typically a few µm
square) of material that it contacts. The apparatus developed by Fryer [91]
employs lower heating rates (of the order of a few degrees Celsius per minute)
because of manual control of the probe set-point temperature and lack of
automatic data logging.
At present, only the analogues of DSC and TMA are commercially available.
Localised dynamic mechanical analysis (DMA) has also been demonstrated
[23,70,72,80,92]. Here, the force between the thermal probe is modulated during
the temperature ramp. Such a procedure may also be used as an imaging mode
in order to obtain a map of variations in mechanical properties across the
specimen [80,92]. Localised modulated-temperature TMA has been performed
whereby the amplitude and phase difference of the modulation in z-axis
displacement of the probe is detected whilst a temperature ramp with overlaid
AC modulation is applied to the tip [81]. An indirect form of thermogravimetry
has been reported whereby the mass of material remaining adhered to the tip was
monitored indirectly using the AC calorimetric signal [70]. By employing
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stiffer cantilevers with integrated heaters, the mechanical resonance frequency
of the beam has also been used to measure the mass of material on the tip during
heating [93].
3.2 Calibration [94]
Localised thermal analysis has generally been used as a qualitative tool; that is,
it is primarily used to identify the material being examined or for examining
compositional gradients within materials. The temperature at which a transition
takes place is often of primary interest, although it may be possible to add
further semi-quantitative interpretation of the data from the magnitude of the
change (e.g. amount of probe penetration) observed. Like all thermal analysis
techniques, interpretation of results requires detailed temperature calibration
procedures and an understanding of the precision of the temperature
measurement.
A good reference material has a number of desirable properties including a
well-documented value, availability in a suitable form for analysis,
homogeneity, stability, low toxicity, and traceability to a national reference
laboratory (NRL). In traditional DSC and TMA, metals like indium, tin, and
zinc meet these criteria. These metals are not suitable for temperature calibration
for localised thermal analysis, however, as they may contaminate the probe tip
thus changing its resistance and defeating the object of calibration..
Organic calibration materials are more suitable for calibration as they do not
react with the probe and the tip may be easily cleaned at the end of an
experiment by heating to above 500 °C in air. This is sufficient to decompose
most organic substances. The British Laboratory of the Government Chemist
(LGC), a national reference laboratory, conveniently offers eleven organic
reference materials with melting temperatures ranging from 41 to 285 °C.
However, in these materials are generally unsuitable to be used directly and
large flat polycrystalline surfaces must be prepared by melting a small amount
of substance in a suitable holder (such as small cup aluminium foil) and then
cooled.
Many investigators, particularly those working in the field of polymer science,
prefer to use films of semi-crystalline thermoplastics such as poly(caprolactone
or polyethylene terephthalate) as melting point standards. Whilst less desirable
from a theoretical standpoint, temperature calibration with polymeric films
offers a number of ease-of-use advantages. Moreover, many polymers have
high melting temperatures that provide a calibration range of nearly 300 °C, a
range difficult to achieve with organic chemicals. Generally, a two point
temperature calibration is carried out spanning the experimental range of
interest. Whatever protocol the investigator adopts, it is essential that the
method is well-documented so that it can be replicated.
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3.3 Features
There are several important differences between the localised calorimetric and
thermomechanical measurements and their more conventional “bulk” analogues.
Firstly, for the calorimetric measurements, the sample size is poorly defined.
This is because the contact area between the tip and specimen is ill-defined.
Furthermore, should the sample soften during the measurement, the probe will
sink into the specimen thus aggravating this effect. Therefore, there is often a
strong correspondence between the shape of the calorimetric response and the
displacement of the probe [95]. A second-order effect, not often considered, is
that the temperature gradient extending away from the heated tip will be affected
by the properties of the specimen and the heating rate employed during the
measurement. This is analogous to the modulated temperature SThM mode of
imaging which has been considered by Pollock and Hammiche [85] whereby the
depth of penetration of the thermal wave emanating from the tip decays more
rapidly for higher modulation frequencies. Slough [96] has considered the case
of a heat pulse lasting the duration of the temperature programme of the probe
for a typical polymeric sample. His calculations suggest that for an
instantaneous heat pulse of 200°C delivered for one second to the surface of a
sample of polystyrene at 25°C, the specimen’s temperature is not significantly
affected 10 µm from the probe. Inoue and Uehara [97] have modelled this
process at a more sophisticated level in the context of data writing and erasing in
a phase-change optical disk. Melting of a crystalline material at a part of the
surface causes surface rippling around the molten area and subsequent rapid
cooling generates an amorphous spot. This amorphous material is maintained at
low temperature and subsequent localised thermal analysis can be used to
determine the glass-rubber transition temperature of this region.
The way in which localised thermomechanical measurements are performed
also differs from conventional measurements. In the usual implementation of
these measurements, the feedback loop between the cantilever force and the z-
axis piezo control is disabled at the start of the experiment. Thus, the probe is
initially applied to the surface with a user-defined cantilever deflection which
will change (thus giving the displacement of the tip by means the optical lever
formed by reflecting a laser spot from the back of the probe into a
photodetector) during the course of the measurement. Should the specimen
soften during the experiment, the probe will indent into the sample and the force
on the tip will decrease (possibly to the extent of losing contact with the
specimen).
A final effect often observed during localised thermal analysis arises from the
high heating-rates that can be employed. Many thermal transitions are governed
by kinetic laws which define the time dependence of such processes as
70
devitrification and thermal degradation. As a consequence of this, some
transitions can appear at elevated temperatures when compared to bulk
measurements at more modest heating rates - even after careful calibration of the
instrument. This effect is not generally seen for melting phenomena, and the
rapid heating affords a means of measuring the melting temperature of
metastable materials without them undergoing rearrangement to more stable
forms [98].
Figure 7. Localised thermal analysis of semi-crystalline poly(ethylene
terephthalate) showing two consecutive measurements at the same location
(solid line is the probe displacement, broken line is the probe power, filled
symbols denote first upwards temperature scan, open symbols denote second
upwards temperature scan, heating rate 10 °C s-1).
Some of the above aspects of the technique are illustrated in Figure 7. This
shows two measurements performed consecutively on the same region of a
sample of poly(ethylene terephthalate) of approximately 40% bulk crystallinity
at a heating rate of 10 °C s-1. During the first upwards temperature scan, very
little effect is seen in the probe displacement until melting of the polymer occurs
around 250 °C. This is mirrored by an increase in probe power which is largely
arises from the increased contact area between the tip and the sample as it
indents the surface. A small decrease in probe power occurs around 125 °C,
well above the "normal" glass-rubber transition temperature of this polymer of
70-80 °C. This feature might be ascribed to devitrification of amorphous
material present. Immediately the first measurement was completed, the tip was
lifted clear of the (now molten) material. The polymer in this area is rapidly
quenched to room temperature by the surrounding specimen. On carrying out a
second scan of the same region, the glass-rubber transition is observed readily in
71
both signals, without any evidence for further rearrangement to crystalline
material above this temperature or subsequent melting of any crystals so formed.
3.4 Terminology
The nomenclature of this new method of analysis is still under development.
The originators (Hammiche, Pollock and Reading) devised the terms
“Calorimetric Analysis by Scanning Microscopy” (CASM) as a name for the
measurement of probe power, and “Mechano-thermal Analysis by Scanning
Microscopy” (MASM) as part of an effort to coin a systematic hierarchy of
terms for this family of techniques [72]. The commercial term for this field is
“µTA” which is a registered trade mark of TA Instruments Inc. (Newcastle,
Delaware, USA) who also holds trademarks on the terms “µDTA” and “µTMA”
corresponding to the calorimetric and thermomechanical methods [99]. Many
authors have used the terms: “micro-thermal analysis” (with or without the
hyphen, abbreviated to “micro-TA”) and corresponding terms “micro-DTA” and
“micro-TMA”. Such usage goes against the recommendations of the
International Confederation of Thermal Analysis and Calorimetry nomenclature
committee who claim that there is ambiguity over the term “micro” because it is
not clear whether the term is related to sample size (mass, volume), the size of
the instrument, or the value of the measured signal (which may be amplified) or
quantity detected [100]. (At least one company markets a “micro-DSC” which
is a high sensitivity instrument for measuring heat flow in the µW range.) The
general term which seems to be gaining popularity is “local(ised) thermal
analysis” and the derivations “local(ised) calorimetry” and “local(ised)
thermomechanical analysis” in order to differentiate this technique from
traditional forms of thermal analysis which measure the global response of the
sample [101]. It this discussion, the term “micro-thermal analysis” (or “micro-
TA”) is used to denote SThM and localised measurements, whereas the
individual measurements are described in full. Until there has been time for the
terminology to develop and become accepted, searching the growing literature
on this subject will be difficult.
3.5 Applications
Since its development and subsequent commercialisation, localised thermal
analysis has been employed in a number of different disciplines. A review by
Pollock and Hammiche [85], summarises the wide range of materials which
have been studied by these techniques. An earlier paper by Price et al. [102]
describes measurements on polymers, electronic components and biological
specimens in the same publication as an illustration of the wide applicability of
72
such measurements. Articles in popular technical publications have also served
to illustrate this point [72, 90, 103-105].
3.5.1 Polymers
Localised thermal analysis has been used for the characterisation of multi-
component polymer thin films [101,109,110] or polymer blends [102,111-113],
for the investigation of surfaces and interfaces between materials [92,98,114-
126], and compositional gradients brought about by the specimen’s processing
history [90,126-131]. In particular, localised thermal analysis is useful for
probing the behaviour of polymers used for the fabrication of microelectronic
components, where the bulk response of the polymer may not be representative
of the same material when it is present in thin layers [126,132-136]. However,
care must be taken to ensure that an apparent elevation in transition temperature
is not due to the substrate acting as a heat sink, thus leading to errors in
temperature measurement [137]. One interesting application of localised
thermal analysis is to use the thermal probe for in-situ processing of materials,
whereby heat is used to cross-link or decompose the substrate [126,138,139].
Again, this technology lends itself to data storage and micro-machining [140].
Figure 8. Localised thermal analysis of multi-layer polyolefin film shown in
Figure 4 [105].
Two examples of the use of localised thermal analysis are provided in order to
illustrate the generic applications of this approach. Figure 8 shows localised
thermomechanical analysis of the surface of the multi-layer film in Figure 4.
Measurements were made at points within this image describing the bulk
73
polymer, the central gas-barrier layer and the thin tie-layer between this and the
bulk film. The melting transition temperatures are consistent with high density
polyethylene, poly(ethylene-co-vinyl alcohol) and medium density polyethylene
for the bulk, gas barrier and tie layers, respectively [101].
Figure 9. Optical micrograph of the cross-section “gel” particle embedded in
a 75 µm low density polyethylene film. Also visible are craters remaining
following localized thermal analysis of the specimen.
Another example is illustrated in Figures 9 and 10 for a blemish or “gel”
particle in a blown polyethylene film. Figure 9 shows an optical micrograph of
the film which has been carefully cross-sectioned across the feature. Also
visible are small craters in the film which are a consequence of a series of
localised thermal analyses along the specimen. These also serve to illustrate the
potential spatial resolution of the technique. Whilst the Wollaston wire probe
has a diameter of 5 µm at the tip and is capable of detecting thermal transitions
in the order of a few square micrometres, heat from the measurement process
spreads out and disrupts a large region (about 20 µm square) around the tip.
This ultimately restricts the proximity of a sequence of measurements on a
specimen. For the analysis of the sample shown in Figure 9 the specimen was
translated under the instrument using a micrometer stage. Over an order-of-
magnitude improvement (both in terms of sampling area and proximal
placement of multiple analyses) can be achieved using semi-conductor probes
such as those shown in Figure 2(c). These have recently become commercially
available and promise to move localised thermal analysis into the nano-scale
with sub-micrometre resolution.
74
Figure 10. Results from localised thermal analysis of the specimen shown in
Figure 9. The solid symbols denote measurements on the normal film whereas
the open symbols denote measurements made in the “gel” particle [103].
Figure 10 shows results measurements made on the bulk film and in the region
of the defect. Whilst it can be observed that the temperatures of the onset of
probe penetration into the low density polyethylene film are broadly similar for
all measurements. The degree of penetration into the sample into the “gel”
particle is much lower. This implies that the material here has a much higher
melt viscosity than the normal perhaps as a consequence of some problem
during polymer synthesis [104].
3.5.2 Pharmaceuticals
The applications of micro-thermal analysis within the pharmaceutical industry
have been reviewed by Craig et al. [141]. Most tablets are not composed of the
pure active ingredient, but contain the drug dispersed within an excipient
package (such as micro-crystalline cellulose, starch, glucose etc.) with added
processing aids (such as magnesium stearate) which act as fillers and lubricants
for the formation of a mechanically stable compact. Furthermore, the tablet
may be coated with a polymer or sugar film to prevent the drug being released
into the body before it enters the stomach [142]. Imaging by SThM can be used
to identify discrete phases containing these ingredients and their identification
can be confirmed by localised thermal analysis [23,69,70, 104,143-147]. Figure
11 illustrates this by means of measurements on the specimen shown in Figure
75
5(a). Localised thermal analysis detects the melting of the drug paracetamol
(acetaminophen) around 180 °C for the low thermal-conductivity region of the
image. No change in response is observed when an area from the high thermal-
conductivity region of the same image is examined. This suggests that the
material here is comprised of a filler or excipient - most probably micro-
crystalline cellulose. The distribution of drug within a pharmaceutical compact
can have very important implications for the dissolution of the tablet within the
digestive system and micro-thermal analysis is a useful characterisation tool.
Figure 11. Localised thermal analysis of the analgesic tablet shown in Figure
5(a) with measurements made in the low thermal conductivity (dark) region
(solid line) and the high thermal conductivity (bright) region (broken line) of the
image [70].
Many drugs can be produced in more than one crystalline modification. Micro-
thermal analysis has been shown to be a viable means of differentiating between
such polymorphs [148,149], or between crystalline and amorphous regions in
drugs [150,151]. Royall et al. [152] have employed localised thermal analysis
to detect surface segregation of progesterone encapsulated in poly(lactic acid)
microspheres (thus confirming earlier work by DSC), while Zhang et al. have
studied the distribution of poly(ethylene glycol) in the same polymer [153].
76
3.5.3 Biology
Micro-thermal analysis has been used to examine a number of specimens of
biological origin. Gorbunov et al. have reported the study of the infrared
receptors of snakes using SThM and localised thermal analysis, whereby the
thermal transport properties of the receptors were found to be lower than the
surrounding skin [65]. The surfaces of plant leaves have also been examined by
localised thermal analysis, to study the waxy coating which protects the leaf
from water loss. Melting of the cuticular wax has been detected [102] and the
effect of surfactant packages on this behaviour investigated as a means of
improving the absorbtion of agrochemicals [154]. Studies of historical
parchment derived from the dermis of animal skin have been employed to
measure the softening temperature of the material. It is a particular advantage of
the technique to be able to examine small quantities of material [155]. SThM
has been used in food science to image the surface of caramel [156]. In this
instance pulsed-force microscopy and infrared spectroscopy were employed to
characterise surface structures, although it would be a natural extension to use
the SThM to study local thermal transitions
3.5.4 Inorganic materials
This category includes metals, ceramics and electronic materials which are
typically of high thermal conductivity whereby the discrimination between
phases afforded by SThM can be compromised [157]. Furthermore, it is
difficult to envisage that point-source heating afforded by active thermal probes
would be sufficient to heat highly-conductive bulk specimens such as metals.
However, thin-film NiTi shape-memory alloys have been studied by localised
thermal analysis, whereby measurements of the martensitic to austenitic
transformation were made and the spatial variation in transition temperature
corresponding to compositional variations within the specimen identified [158].
Micro-thermal analysis has also been employed to resolve differences in thermal
conductivity and softening temperatures that arise during the processing of
carbon fibres [67,129,159]. These were related to the local oxygen content of
the fibre measured by electron probe micro-analysis.
77
Figure 12. Shaded topography (left) and thermal conductivity contrast (right)
images of a light emitting diode. Reproduced from reference [102] with the
permission of Akadémiai Kiadó.
Figure 13. Localised thermal analysis of the LED shown in Figure 12. The
upper set of curve show measurements made within the high thermal
conductivity central region and the lower set of curves show measurements
made outside this area [102].
As described earlier, passive SThM techniques have proven popular in the
microelectronics industry for the identification of hot-spots within components.
For example, Boroumand et al. have made measurements of the temperature
distribution across a polymer light-emitting diode (LED) [160]. Figure 12
shows the topography and thermal conductivity contrast image of a silicon-
based LED from a batch of components which had failed under testing.
78
Localised thermomechanical measurements of the centre and outside of
specimen are shown in Figure 13. Although no thermal transitions are observed,
the thermal expansivity is different between the two areas. This would lead to
thermal stresses building up when to the device was energised. This could
ultimately lead to failure of the LED. Measurements on a LED which passed
testing showed no variation in properties [102,104].
4. LOCALISED CHEMICAL ANALYSIS
The measurements physical properties afforded by existing forms of micro-
thermal analysis can be insufficient to discriminate between different materials.
Incorporation of some means of chemical analysis of the specimen is therefore
highly desirable. This has been achieved by two processes; localised pyrolysis-
evolved gas analysis, and near-field photothermal spectroscopy.
4.1 Localised evolved gas analysis
It has been demonstrated that the Wollaston wire probe tip used for localised
thermal analysis measurements can be heated rapidly and repeatedly to
temperatures in excess of 600°C. This is sufficient to bring about localised
pyrolysis of most organic materials [72,157]. This process generates a small
plume of gaseous decomposition products characteristic of the substrate.
Chemical analysis of these products can be performed via two alternative routes:
by absorbing them on a suitable substrate and subsequent investigation using
thermal desorption gas chromatography-mass spectrometry (td-GC-MS), or by
direct sampling by mass spectroscopy (MS) [23,70,106,162-165]. Furthermore,
the thermal probe may be used to soften and remove material from the surface of
the specimen for subsequent characterisation [70]. These approaches are
described in more detail in the following sub-sections.
4.2.1 Offline localised pyrolysis-td-GC-MS
Realisation of the first approach – trapping and offline analysis – employs a
miniature gas-sampling tube packed with a mixture of Tenax (molecular sieve)
and Carbopak (activated charcoal) absorbent material. Such tubes are
routinely used for environmental monitoring of hazardous industrial
atmospheres whereby operators during the normal course of their duties carry a
small tube (about the size of a pen) clipped to their clothes. A pump may be
used to draw gas through the tube at a controlled rate and, at the end of the work
period, the tube is sealed and sent for analysis. Heating the sorbent tube drives
79
off the trapped material into a gas chromatograph for separation and
quantification.
For micro-pyrolysis-td-GC-MS, the sorbent tube is modified to end in a short
section of stainless steel hypodermic tubing the open end of which can be placed
immediately adjacent to the heated thermal probe using a micro-manipulator.
As the tip is heated, a pump is used to draw gas through the tube. After
sampling, the tube is placed in a suitable carrier that fits into a standard thermal
desorption unit interfaced to a GC-MS system. Blank desorption runs of the
sorbent tube are carried out before and after each pyrolysis experiment to
confirm the cleanliness of the detection system. Such tubes are re-usable since
the thermal desorbtion process cleans them of trapped material. The lifetime of
such tubes is at least 1000 cycles.
A dedicated design of micro-manipulator is used for positioning the sampling
tube which can be interfaced with a variable-temperature sample stage upon
which the microscope is placed. Thus, the sample can be cooled or heated
independently of the thermal probe, using a small heated sample holder coupled
to a Dewar vessel containing liquid nitrogen for cooling the specimen.
Generally, the specimen is only required to be under ambient conditions.
Therefore, an x-y-z translator can be used in place of the heated stage. This
configuration is very versatile and affords independent positioning of the
sampling tube, microscope and sample.
One of the obvious benefits of off-line trapping and analysis of pyrolysis
products is the ability to take samples of evolved gases from more than one
location on the sample. For example, if the region of interest covers a
sufficiently wide area, then multiple points may be selected for pyrolysis and the
evolved gases gas can be trapped in the same tube, thus increasing the yield of
material for subsequent analysis. Alternatively, line or area scans of surfaces
may be made with a heated probe to drive off any volatiles into the tube [165].
This approach has particular benefits for filled systems, such as paints and
coatings, which may contain only a small fraction of organic binders.
Another benefit of analysis by GC-MS is to use the ability of gas
chromatography to separate the mixture of decomposition products yielded by
all but the simplest of substrates. This allows complex systems (such as paints
and coatings described above) to be identified or at least “fingerprinted” by the
characteristic mixture of volatile materials formed during thermal degradation
[166]. Alternatively, one may only be interested in the presence (or absence) of
a particular component at a specific location. For example, this technique has
been used to locate the source of camphor extracted from plant leaves [164].
It is possible to exploit this technique as a means of surface-specific pyrolysis.
For example, Figure 14 shows total-ion chromatograms for the decomposition
products from samples of the same household paint. The top curve (“macro-
80
EGA”) shows material collected from a bulk sample (about 1 mg) of paint using
a thermobalance to decompose the material and gas sampling tube in the purge
gas outlet to collect the evolved gases. The lower curve (“micro-EGA”) shows
material collected by scanning a hot thermal probe over at 25 × 50 µm area of
the surface of the paint. Both curves show common features arising from the
binder in the paint (polyamide and acrylic polymers), but also differences
between the surface and the bulk indicating a reduction in antioxidants and
volatile plasticisers (particularly the peak at 21.3 min due to dibutyl phthalate) at
the exposed surface [165].
Figure 14. Comparison of evolved gases from bulk (macro-EGA) and surface
(micro-EGA) of paint film [165].
4.2.2 Online localised pyrolysis-MS
As an alternative to analysis of evolved gases by GC-MS, mass spectroscopy
by itself can be used. This has the disadvantage of lacking the specificity given
by the chromatographic separation, but the advantage of considerably reducing
the time for analysis. One of the drawbacks of the td-GC-MS approach is the
time taken for the analysis of collected gases from the micro-pyrolysis
experiment. The thermal probe itself may be heated at over 100°C/s to the
required pyrolysis temperature and several locations may be examined within a
few minutes. Desorption and separation of the trapped material is limited by the
cycle time of the GC-MS system. Even with an optimised oven programme for
81
the GC, a typical analysis can take 30 minutes. Furthermore, it is advisable to
conduct a “blank” experiment with the tube prior to sampling in order to ensure
that no residues are present in the system.
One way around this problem is to dispense with the trapping and separation
stage by continuous sampling of the atmosphere around the thermal probe. This
can be achieved by using a small-bore silica-glass capillary transfer line which
also serves to reduce the pressure from atmospheric to that which could be
accommodated by the ion source of the mass spectrometer. The capillary tube
must be surrounded by a heated jacket for most of its length in order to prevent
condensation of volatiles on the walls of the tube. Only a few centimetres of the
end of the capillary are left unheated – these were passed through an empty
micro-sorbent tube (described above) so as to enable the same micro-
manipulator assembly to be used to position the end of the transfer line close to
the thermal probe.
For online micro-pyrolysis-MS, three modes of pyrolysis have been developed.
Firstly, the temperature of the probe may be rapidly pulsed to the required
temperature – the amount of material liberated depending upon the duration and
temperature of the heat pulse. Secondly, a conventional temperature ramp can
be applied to the probe whilst monitoring for evolved species. This approach
has the benefit that the operator can select several locations within the field of
view of the microscope in order to carry out compositional mapping via evolved
gas detection. Finally, a heated thermal probe may be brought into contact with
a specimen whilst monitoring gas evolution. As the heat source nears the
surface, material is progressively decomposed, affording a means of depth-
profiling though the sample so as to reveal buried layers beneath the surface
[106,164]. Alternatively, successive pyrolysis measurements may be made
using either of the first or second methods of heating the probe with either direct
sampling to the mass spectrometer, or the more time-consuming offline td-
GCMS sampling [106].
As stated earlier, online MS sampling of evolved gases lacks the separation
stage afforded by td-GC-MS and is unsuitable for complex systems.
Furthermore, the sensitivity of the technique is improved by monitoring for
specific species rather than collecting mass spectra across a wide mass range
during pyrolysis. For example, materials containing aromatic species often
evolve benzene amongst their decomposition products, whereas aliphatic
materials often give alkene fragments. These may be detected by monitoring
single-ion masses rather than acquiring a full mass spectrum. The ability to
examine a succession of points within a few minutes can enable a compositional
map of the specimen to be obtained, which shows the spatial distribution of
phases in a specimen via its pyrolysis products [165].
82
An example of this is shown in Figure 15 for a poly(methyl
methacrylate)/polystyrene laminate. Data from a 6 × 6 array of pyrolysis
measurements were used to reconstruct images based upon the ion yield of the
respective monomers. Unlike similar methods of chemical imaging (e.g.
secondary ion mass spectrometry and laser ionisation mass spectrometry), the
sample is examined under ambient conditions rather than high vacuum [167].
Three-dimensional tomographic imaging may also be considered by using the
probe to ablate the surface.
Figure 15. Left: topographic image of poly(methyl methacrylate)/polystyrene
laminate – polystyrene layer is to the right of the image. Centre: evolved gas
“image” reconstructed from multiple pyrolysis measurements monitoring for
methyl methacrylate monomer (molecular ion m/z 100). Right: similar image
reconstructed for styrene (m/z 104) [165].
4.2 Near-field photothermal spectroscopy
When a material is illuminated with infrared radiation it will tend to absorb
energy corresponding to specific infra-red (IR) active modes and increase in
temperature. This is known as the “photo-thermal effect” and can be exploited
to obtain the IR spectrum of a specimen. Passive temperature-sensing SThM
probes can be used to measure the specimen response to IR radiation over a very
small area below that realisable using conventional optics because the spatial
resolution is limited largely by the contact area of the tip and the thermal
diffusivity of the specimen being of the order of 1 µm which is at least one
order-of-magnitude improvement over conventional techniques. Near-field
photothermal spectroscopy using resistive probes of the types shown in Figure
2(a-b) has been exploited by Hammiche and co-workers [40-47], who used a
mirror system to bring the IR beam to focus on the specimen surface while it
was in the SThM. Two approaches have been used to generate the spectrum. In
the first method a high-intensity tunable IR source has been used to scan the
spectrum in a manner analogous to dispersive IR spectroscopy [42]. The
83
second approach uses a broadband source in an interferometer and subsequent
Fourier transform analysis to obtain the spectrum [40].
The results of this method, in addition to the work described above, have also
been combined with measurements by SThM and localised thermal analysis.
Polymers [106] and pharmaceuticals [23] represent the largest classifications of
materials that have been investigated, although this technique has been used in
cellular biology to monitor the life-cycles of cells [168].
4.3 Thermally-assisted micro-sampling
Figure 16. Online MS single-ion monitoring for methyl methacyrlate
monomer (using the CH2C(CH3)CO.+ ion fragment m/z = 69) removed from the
surface as a consequence of heating the probe in contact with the specimen.
The ability of the heated thermal probe to soften the surface of a specimen and
remove a small amount of material has also been demonstrated. By placing the
probe on the sample and raising its temperature, the substrate is softened [70].
As the probe is withdrawn from the sample, some material adheres to tip. This
residue can then be characterised by pyrolysis-evolved gas analysis (td-CG-MS
or MS). Figure 16 illustrates this process using online-MS monitoring for the
evolution of monomer from a specimen of poly(methyl methacrylate). The tip is
first heated rapidly to 1000 °C to ensure that it is free of any contaminants.
Then, the probe is brought into contact with the specimen and the tip is heated to
84
400 °C at 10 °C s-1, whereupon a small amount of monomer is evolved as the hot
tip is removed from the surface. A second probe-cleaning cycle results in a
larger amount of monomer being detected as a result of the decomposition of
material adhering to the tip. Subsequent cleaning of the tip by heating to 1000
°C does not result in the detection of any residue. Experiments indicate that it is
only necessary to heat the specimen above its glass-rubber transition or melting
temperature in order to transfer material from the surface to the tip. Analogous
measurements have been demonstrated using photothermal IR spectrometry to
detect material removed in this way [169]. Thermal dip-pen nanolithography
using a heated AFM tip has also been developed [170]. In this case, the tip is
coated with an organic material which is transferred to the surface by heating the
tip so as to melt the “ink”. This is essentially the inverse of the micro-sampling
process described above.
5. CONCLUSIONS
This chapter presents an overview of micro-thermal analysis. Scanning
thermal microscopy has gained acceptance in many areas of physical science.
The commercial availability of instrumentation will continue to broaden its
scope of usage into more areas of characterisation of materials. A better
understanding of the mechanisms of heat transport from the tip to the surface
can be expected to make routine measurements of absolute thermal properties
via scanning thermal microscopy possible. The thermodynamic limit of
measurement (kT) is about 10-21 J at room temperature [27]. The current spatial
resolution of scanning probe microscopy is around 10-10 m. The maximum
temperature resolution of the most sensitive thermal probes (bimetallic
cantilevers) is 10-5 K with an estimated sensitivity limit of ≈10
-12 J and a spatial
resolution of ≈10-7 m. There is, therefore, plenty of room for improvement.
Advances in thermal probe design are also expected to lead to applications for
localised thermal analysis, thus enabling the behaviour of materials to be
investigated over even smaller dimensions. The integration of chemical analysis
(by pyrolysis and/or near-field photothermal infrared microscopy) raises the
possibility of constructing a versatile instrument capable of performing a wide
range of analyses that exploits abilities of the thermal probe to act as a very
small heater and thermometer. Such an instrument might well be termed “the
laboratory on a tip”.
85
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