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HOPG Substrate Materials for Scanning Tunneling Microscopy and Atomic Force Microscopy

hopg
hopg

SPI Supplies offers HOPG from two entirely different sources. It is not a matter that one is better than the other, the quality of both is excellent. However, the two HOPG Brands are made using different processes, and therefore their physical characteristics are a bit different. SPI is happy to present both types of HOPG and let our customers decide which will work best for them.

Introduction

HOPG, is a relatively new form of high purity carbon and provides microscopists with a renewable and smooth surface. Unlike mica, HOPG is completely non-polar, and for samples where elemental analysis will also be done, it provides a background with only carbon in the elemental signature. The extreme smoothness of HOPG gives results in a featureless background, except at atomic levels of resolution. The modern-day material known as HOPG can be traced back to what at one time was called "Kish graphite". The SPI Supplies brand of HOPG should not be confused with the SPI Supplies brand of glassy carbon.

Lamellar structure

Graphite in general and HOPG in particular are described as consisting of a lamellar structure, like mica, molybdenum disulfide and other layered materials that are composed of stacked planes. All of these examples of lamellar structures have much stronger forces within the lateral planes than between the planes, thus explaining the characteristic cleaving properties of all of these materials.

Some interesting information about HOPG

Cleaving Properties

HOPG because of its layered structure cleaves almost like mica. The usual approach is to take a piece of tape (e.g. 3M® "Scotch Brand" double sided tape), press it onto the flat surface, pull it off and the tape invariably takes with is a thin layer of HOPG. This freshly cleaved surface is what is used as sample substrate material. How many cleavings per sample? This is not easy to answer, but per a 2 mm thick block of the best grades (e.g. SPI-1 or ZYA), it is reported that one can get 20-40 cleavings. We do not guarantee a fixed number of cleavings. For the lower level grades the number of cleavings per 2 mm thickness will be less, but again just how much less we can not predict with accuracy.

Basal Plane Image

In an atomic resolution scanning tunneling microscope image of the graphite structure of HOPG, there are two possible images. The image normally obtained looks like a close packed array; in this array each atom is surrounded by six nearest neighbors. The distance between any two of these atoms is 0.246 nm. Under ideal conditions, particularly if the probe tip is truly a single atom, you will see the "chicken wire" structure that shows the hexagonal rings that are the true structure of graphite; the center to center atomic distance in this image is 0.1415 nm. This distance in either case is an atomic property of carbon and it does not depend on the grade of graphite. The image shown here is the close packed array and that is the image obtained form the basal plane of HOPG under most circumstances.

Use as graphite monochromators:

The HOPG offered by SPI Supplies, especially the SPI-1 and ZYA grades, are reported to diffract x-rays and neutrons with a higher efficiency that any other material. For x-rays the intensities are increased up to five times greater than what would be possible with the lithium fluoride crystal it would replace. A singly bent focusing monochromator using these highest quality grades of HOPG, i.e. those with the smallest mosaic spread, result in intensities three times that of LiF at the same resolution.

Flatness/Roughness

The HOPG has a structure of polycrystals, the size of which varies, the maximum being 10 mm for the highest quality. The freshly cleaved surface consists of atomic steps, 0.2-0.3 nm and steps of several or dozens of atomic layers. The higher the quality, the less the roughness of the surface, and the smaller the number of such steps on a freshly cleaved surface.

Mosaic spread

This term is a measure of just how highly ordered is the HOPG. The lower the mosaic spread, the more highly ordered is the HOPG, resulting in a cleaved surface that exhibits virtually no steps. Lower mosaic spreads are also associated with higher prices. However, since the lower mosaic spread material is more "cleavable", one usually realizes more "cleavings" from the more expensive material, so the increased cost is a bit less than it might first appear.

This is where it gets a bit confusing: There are two distinctly different "brands", the SPI Supplies Brand and the GE Advanced Ceramics Brand. The processes used to manufacture HOPG are understandably kept quite secret but we have reason to believe that they are quite similar, at least they are much more alike than not alike. Available from both brands are three grades, the top one being the "calibration" grade, the second being "research" grade which is acceptable for most experiments and a "technical" grade where less demanding material is needed, perhaps when one is mainly interested in demonstrating cleavage properties.

The classical way of measuring mosaic spread is by x-ray diffraction, however, there are different methods reported in the literature and as one might expect, different measuring methods yield somewhat different numbers. The precise details of the x-ray diffraction methods used for the characterization of the HOPG from the two different sources is not disclosed publicly and therefore, and without commenting on which way is the better of the two, we must suffice it to say that comparing the numbers generated, one method to the other might be like comparing apples to oranges. However, we will touch upon what can be disclosed about the differences in the measurement methods below.

Columnar structure:

The structure is strictly columnar, that is, the columns run vertically within the flat slab of the material. The grain boundaries can be seen on the lateral surfaces. The mosaic spread is the angle of deviation of the grain's boundary from this perpendicular axis (of the columnar structure). Researchers in laser physics found this aspect of the information important.

Use at elevated temperatures

As more and more applications are found for HOPG in research and technology, more and more applications are requiring good high temperature characteristics. We can report the following information which should be useful for those contemplating such usage:

Calibration in z direction:

The hills and valleys on a cleaved HOPG surface are not calibrated as to their height. However, the crystallographic planes do have a definite structure and the height of a single step is 0.34 nm. Reference: L. Pauling, The Nature of the Chemical Bond, p. 235, 3rd. Edition 1960.

You may find steps randomly as a result of the cleavage process. An alternative method is to create etch pits by oxidizing the surface in an oven in air.

High Purity:

All HOPG grades have comparable purities and impurity levels are on the order of 10 ppm ash or better.

Chemical inertness:

HOPG exhibits high chemical inertness to just about everything including osmium tetroxide. The one environment, however, where it will "disappear" quickly is in the presence of an oxygen plasma of the type generated in the SPI Supplies Plasma Prep II plasma etcher.

Thermal conductivity:

Because of the anisotropic nature of HOPG, the thermal conductivity is different in different directions. It is 1800Wt/C° along the basal plane, and 8-10Wt/C° in the direction perpendicular to the basal plane. Thermal conductivity is high for any type of HOPG. Heat transfer HOPG has the same thermal conductivity as other HOPG samples, but is cheaper. The thermal conductivity seems to be independent of HOPG grade.

Density of HOPG:

The density for all three grades (SPI-1, SPI-2, and SPI-3) is 2.27 g/cm-3.

Measurement side:

X-ray diffraction is done on only one of the two sides of each HOPG plate and the value obtained is the value reported. The two sides are not exactly the same, and there can be a slight asymmetry through the cross-section of each plate. When the HOPG is manufactured, one side ends up relatively flat looking and the other size somewhat "bubbly". Believe it or not, the tightest mosaic spread is the side that is "bubbly". The opposite side will be either the same mosaic spread or slightly less tight. We are changing over the packaging of the HOPG of the SPI Supplies brand so that there will be a bit of tape on that "opposite" side so that there could never be any question as to which side is which.

Ferromagnetism:

Some low levels of ferromagnetism has been observed in HOPG. This not a contamination issue but rather due to the highly ordered structure of HOPG, as compared to natural graphic which does not have such a structure.


Two Different HOPG Sources are available for SPI Supplies customers

SPI Supplies

SPI Supplies offers its own "brand" of HOPG. It comes in three different "grades" as explained above. It also comes in various "standard" sizes and one benefits from quantity production economics by purchasing these standard "off-the-shelf" sizes.

Density of HOPG:
The density for all three grades (SPI-1, SPI-2, and SPI-3) is 2.27 g/cm-3. We do not know the density for the Advanced Ceramics brand of HOPG but we believe it to be very similar if not the same.

Measurement side:
X-ray diffraction is done on only one of the two sides of each HOPG plate and the value obtained is the value reported. The two sides are not exactly the same, and there can be a slight asymmetry through the cross-section of each plate. When the HOPG is manufactured, one side ends up relatively flat looking and the other size somewhat "bubbly". Believe it or not, the tightest mosaic spread is the side that is "bubbly". The opposite side will be either the same mosaic spread or slightly less tight. We are changing over the packaging of the HOPG of the SPI Supplies brand so that there will be a bit of tape on that "opposite" side so that there could never be any question as to which side is which. See below for more comment on what this all means.

Grade SPI-1
Very similar to Grade ZYA described below and is comparable to the very best "calibration grade" HOPG and exhibiting a 0.4° +/- 0.1° (demonstrating outstanding crystalline perfection) but lower in price than other HOPGs of comparable quality. This is the most highly ordered, lateral grain size is typically up to about 3 mm but can be as large as 10 mm, and is used primarily for instrument calibration purposes or for research experiments where for some reason, the very ultimate in HOPG order is needed. This is the highest quality material and also the most expensive. Suitable for the most critical calibration requirements and research applications.

Grade SPI-2
Very similar to ZYB(see below), this grade exhibits a mosaic angle as small as 0.8° +/- 0.2°. This grade is slightly less highly ordered than SPI-1 but is acceptable for most users. The lateral grain size can be up to 0.5 mm but can be as large as 1 mm. Note that this grade is slightly less "cleavable" than the 0.4° mosaic spread product, that is, SPI-1. Most workers find this grade more than suitable for all but the most critical samples, but priced low enough for routine use. Some users prefer this grade to SPI-1 because the cleavage steps are slightly wavy, so samples are held in place better (e.g. better adhesion).

Grade SPI-3

Exhibits a mosaic angle as small as 3.5° "+/- 1.5° and while it might not be acceptable for the most demanding of experiments, for student use its much more economical pricing makes it ideal. This grade is much less highly ordered and has a grain size not larger than the range of 30-40 nm. It is also excellent for student use. Because of the lower level of order, one can expect that the individual "strippings" will exhibit some population of "steps". Thickness may be 0.5 mm.

Thickness specification: (Grade SPI-1 and SPI-2 only)
The standard specification is 0.1 mm. This thickness is well beyond what is needed for a typical x-ray monochromator. However, we do want to make the point that the tolerances on the thickness dimension are quite small compared to other brands and sources of HOPG. And the customer is never going to end up with anything thinner than 1.9 mm for a nominal 2 mm thick product.

Electrical resistivity: Parallel to layer plane: 4 x 10-5 ohm-cm
Perpendicular: 0.15 ohm-cm

Cutting into other sizes:
We can provide custom HOPG depending upon a customer's requirements. Please note that we are unable to provide custom HOPG discs at this time. For custom HOPG quotations, please contact us contact us.

Thin HOPG strips also available:
HOPG thin strips are available only in the SPI-1 Grade. (Thickness = 0.2 mm 0.1 mm.)

3 mm Round Discs
Available in Grade SPI-3 and Grades SPI-1. Thickness is 50-70µm.

When statement of thickness don't always mean what they say........
For some time we have offered SPI-2 and SPI-3 in (nominal) 2 mm thickness but because of some recent changes we have made, we find that what we have been calling 2 mm could actually be as thin as 1.6 mm. So we continue to call the product 2 mm, but it is a "nominal" thickness and could be less thick. Our 1 mm and 3 mm products are indeed quite close to the stated thickness.

MSDS RoHS Button
SPI-1 Grade square   Squares and Rectangles

Momentive Performance Materials
The Advanced Ceramics brand of HOPG was originally produced by Union Carbide Corporation and then became Advanced Ceramics. Advanced Ceramics became part of General Electric Company and then, in 2007, became Momentive Performance Materials. The product is the same product, made by the same people, despite the name changes. Advanced Ceramics HOPG comes in three different grades, historically known as ZYA, ZYB and ZYH, as described below. This was the "original" brand of HOPG that the original researchers with SPM technology used for the development of the techniques and methods used throughout the world today.

Measurement of the Mosaic Spread:
We have been told by the manufacturer that the method used is x-ray diffraction and that for each and every single piece of Advanced Ceramics HOPG that is sold, there have been not less than four different measurements made on each of the two sides. Clearly this level of measurement goes beyond what is done for the SPI Supplies brand of HOPG. We have also been told that there has never been any evidence that there is any difference in the mosaic spread, one side of the plate to the other.

GRADE ZYA
The highest quality HOPG, ZYA is intended for instrument calibration but some researchers prefer it for the most critical of research samples. Virtually no steps seen are on the cleaved faces. Mosaic spread angle is 0.4° +/- 0.1°.

GRADE ZYB
Suitable for most research applications when samples are to be mounted samples for analysis. It is also easily renewed and the user should be able to get fifteen to twenty cleavings per HOPG block. Mosaic spread is 0.8° +/- 0.2°.

GRADE ZYH
While maintaining the same level of purity as the other grades of HOPG this grade has cleaving properties that are somewhat less than that of the other low grades. Ideal for non-critical work, also the perfect low-cost alternative to the classroom environment and student use. Mosaic angle is 3.5° +/- 1.5°fmease.

Electrical resistivity:
msds

SPI-1 Grade
DimensionsSPI #Each10+In Stock
5 mm x 5 mm x 1 mm476HP-AB$255.57$230.01 Add to cartYes
7 mm x 7 mm x 1 mm498HP-AB255.57230.01 Add to cartYes
10 mm x 10 mm x 1 mm439HP-AB255.57230.01 Add to cartYes
10 mm x 10 mm x 2 mm438HP-AB425.23382.71 Add to cartYes
12 mm x 12 mm x 1 mm426HP-AB396.96357.26 Add to cartYes

Thin strip HOPG:
10 mm x 10 mm x 0.2 mm424HP-AB$107.72$96.95 Add to cartYes


3 mm discs
3mm diameter fit into a standard TEM holder. 10-40µm thick
Pack of 20 discs
3mm diameter
10-40µm thick
425HP-CA320.86288.77 Add to cartNo
Pack of 100 discs
3mm diameter
10-40µm thick
425HP-MB1,000.00900.00 Add to cartNo



SPI-2 Grade
DimensionsSPI #Each10+In Stock
5 mm x 5 mm x 1 mm479HP-AB$105.42$94.88 Add to cartYes
7 mm x 7 mm x 1 mm480HP-AB105.4294.88 Add to cartYes
10mm x 10 mm x 2 mm
(can be as thin as 1.6 mm)
446HP-AB170.75153.68 Add to cartYes
10mm x 10 mm x 3 mm 456HP-AB404.83364.35 Add to cartYes
20mm x 20 mm x 1 mm466HP-AB408.27367.44 Add to cartYes
20mm x 20 mm x 2 mm
(can be as thin as 1.6 mm)
444HP-AB679.74611.77 Add to cartYes


3 mm discs
3mm diameter fit into a standard TEM holder. 50-75µm thick
Pack of 20 discs
3mm diameter
50-75µm thick
421HP-CA240.65216.59 Add to cartNo
Pack of 100 discs
3mm diameter
50-75µm thick
421HP-MB800.00720.00 Add to cartNo
DimensionsSPI #Each10+In Stock
ZYA™
12mm x 12 mm x 2 mm
(can be as thin as 1.6)
430HP-AB$993.48$894.13 Add to cartYes
ZYB™
12mm x 12 mm x 2 mm
(can be as thin as 1.6)
432HP-AB993.48894.13 Add to cartYes
ZYH™
12mm x 12 mm x 2 mm
(can be as thin as 1.6)
434HP-AB132.65119.39 Add to cartYes