Your substrate literally
is the foundation of good results. Here are some things to consider
when choosing a substrate.
Substrate Considerations Substrate choice can be very crucial in microscopy. In some cases,
the wrong substrate can make a given microscopy technique impossible.
Some things to consider when choosing a substrate:
Conductivity
Some techniques require a pathway for electrons to and from the sample,
e.g. SEM and STM. In such a case, mica, an insulator would likely be
a poor choice. However conductive coatings, such as a gold film can
make the surface usable. Be sure to know if your microscopy has special
needs in this regard.
Smoothness/Roughness
The desired technique as well as the desired data can both require
varying degrees of flat clean surfaces. How smooth does your substrate
need to be?
Sample-Substrate Interaction
Chemical reactivitiy and mobility are common concerns as well as whether
or not your sample will even stick to the substrate. If you are using
spin coating to deposit your sample, then sticking is even more important
of an issue.
Common Substrates
Gold Films
I have worked almost exclusively with gold on mica thus far, but gold
films are also commercially available on glass (see below).
- Pros
- Atomically flat terraces; per Molecular
Imaging, "Atomically
flat terraces are typically several
hundreds
of nanometers
across, with this size
increasing after
hydrogen flame annealing."
- Cons
- Mica is not a fun material to cut. It is easy to unintentionally
delaminate the mica, deform it, and/or make the gold film unusable.
Take a look at cutting mica, below. Because
of the gold film, the punch method is not very useful, due to
contamination (when you see a punch, you'll understand).
Vendors
- Arrandee - These folks specialize
in gold on glass. Nota bene: Hydrogen flame annealing is too hot for
these substrates.
- Platypus Technologies, LLC
- They specialize in gold-coated substrates, providing gold on mica,
glass, and silicon wafers.
- Molecular Imaging - Makers of
SPMs and gold films on mica.
HOPG
I found useful info on HOPG use to be quite sparse so I thought I would
type up what I have found.
Vendors
You need to buy some
before you can use it. Following are the
only vendors I know of thus far:
Pros
- HOPG can be cleaved to a nice, clean surface.
- It is conductive
- It is made of carbon
Cons
- It is quite expensive, about hundred-fold more expensive than mica
- Lead times for orders are usually at least a week
- Very few vendors which leads to a lack of competitive salesmanship
that fosters good customer service
Preparation
- Cutting
- Life is easier
if you can avoid this, but the best method seems to be
a clean, sharp razor blade. I later was told that's
how Advanced Ceramics does it. You will likely introduce clumping
at the edges and shear forces that will split the HOPG along the
planes, producing multiple, thinner pieces of HOPG. The less
cutting a specific piece undergoes, the better. I
have successfully cut a piece on all four sides and
used it, but it required a lot
of finesse, patience, and tape (of course, I wanted
a double-sided cleaving as well).
- Cleaning one side
- HOPG is a layered
surface which you can cleave using simple scotch tape. Unlike
mica, some HOPG samples seem to have a distinct preference
for which direction will provide the best cleaving.
- Cleaning both sides
- This can be
irritating. There are two approaches I have
had reasonable success with:
- use two pieces of tape simultaneously, cleaving one surface
and then the other
- basically cleave each side in succession:
- secure a glass slide to a surface
- put a piece of sticky tape on it with a tab hanging over
the edge of the glass
- put the HOPG on the tape
- use a second piece of normal scotch tape to cleave off the
top surface
- pull up the tape and repeat
- Note: the second time around, you may want to carefully scrape
the the sticky tape up with a razor blade so that you can cleave
the underside cleanly again
- Thickness
- Once the HOPG get to the order of ~0.1 mm thick, it starts to
become to flexible and cracks and defects due to simple bending
of the substrate become very problematic.
Mica
A mineral formed in flat layered sheets.
- Pros
- Cheap
- Easy to cut into desired shapes
- Easy to clean via simple cleaving with scotch tape
- Cons
- A fair amount of mica is too poor a quality for some uses
- Solvent can get trapped in the substrate
- Brittle, thin material
- Insulating - inadequate, as is, for some applications, e.g.
STM
- Conductivity: Mica is an insulator, although it is often used
as a substrate for a conductive layer, esp. gold films. Even with
such films, attention needs to be paid to making sure the conductivitiy
needs of your technique, if any, are met.
- Smoothness: RMS roughness of < 1 nm over one square micron
is easily achieved.
Cutting Mica
Cutting mica can be problematic. There are two basic approaches
that I have found viable, using a sharp straight blade to make a
guillotine-like cut or a precision punch. If you have good advice
and/or suggestions, please do not hesitate to contact
me.
Guillotine Cut
To make a guillotine-like cut, you simply have to arrange the
system so that the blade cuts through at a slight angle away from
the surface. I have actually used a simple hobby tool for this,
the Chopper. Note: this is still very much an art. You can buy one of these via most Railroad hobby shops, or online hobby sites like micromark.com.
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It consists of a standard razor blade affixed to a metal lever.
By placing a thin teflon block under the blade, a guillotine-like
cut can be approximated reasonably well. A slight curl sometimes
results., but with practice the frequency of this problem can be
minimized.
Note: some folks have good luck with using a fresh razor blade
flat against the surface, but the occurence of delamination seems
greater.
Punches
Quality punches are great for making discs of mica for AFM imaging.
Incredibly easy and quick without delamination. Surface contamination
is certain, but with mica, the ability to cleave off both surfaces
to produce a clean surface makes this problem moot. I have found
the hand held punches from Ted Pella, Inc. produce a messy disc,
but of course, they offer nice clean mica discs for sale. Clearly,
this option is not as desirable for gold-coated mica as you may
rip the gold surface off or damage/contaminate it beyond utility.
Vendors
- Ted Pella, Inc. - They
sell V1 and V2 (a higher grade); use their site search engine to
find it. I have good dealings with these folks.
- B & M Mica Co., Inc., P.O. Box 525243, Flushing, NY 11352,
USA. Tel: 718-461-6233, Fax: 718-461-0414. 0.0001" Thick & Up
(need to call and verify)
MoS2 (Molybdenum
disulfide)
This material can be a
real nuisance to work with, because it is a thin flimsy film. I
have worked with it too little to offer up more information right
now.
Vendors
Sapphire (Al2O3)
Sapphire is single-crystal Al2O3.
- Pros
- flatness - as low as 3 Å roughness (Ra) [Wafer
World]
- transparent - handy if you wish to backside illuminate
the system (lasers, etc.)
- hardness - Moh's hardness of 9 (out of 10); it can
cut glass.
- Melting point - 2320 K (softening point 2070 K)
- Cons
- hardness - (see above); it can only be cut by diamond
- somewhat expensive
- may require dicing service
Information Links
Cleaning Sapphire - I use a piranha solution to clean the wafers
and I had good results; see my cleaning
page for details.
Silicon
- Pros
- Well characterized
- History of use, which makes protocols related to it
easier to find
- Robust, not as brittle as mica, for example
- Cons
- Difficult to cut into desired shapes, unless your
desired shapes are rectangular
- Good cleaning is time-consuming
- Conductivity: Silicon is a semiconductor. Doping can
have profound effects on conductivity, as can the thickness,
if any, of the oxide layer. You should get this number
from your manufacturer.
- Roughness: This will depend on your processing methods,
if any. Generally, < 1 nm RMS roughness over one square
micron is easily achieved.
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