Fahlman B.D. Materials Chemistry

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8. Soak the copper plate and nickel-ﬁlled membrane in a beaker of acetone for

5 min to dissolve the electrical tape’s adhesive. Carefully detach the membrane

from the copper plate and the electrical tape. If the copper plate is discolored, it

can be cleaned with nitric acid before use in another deposition.

9. To free the nanowires from the alumina template, the GaIn or silver cathode

must be removed and the alumina membrane dissolved. To remove the cathode

layer, mount the membrane with the GaIn or silver side facing outward on a

glass microscope slide using one piece of electrical tape on the support ring. Dip

a cotton-tipped stick in concentrated nitric acid and rub it over the surface once

or twice. The GaIn or silver will dissolve as it is oxidized and the membrane will

appear black as the nickel underneath is exposed.

10. Immediately soak the cotton-tipped stick in water to prevent formation of nitro-

cellulose. Rinse the membrane surface many times with deionized water to

remove excess acid. Peel off the electrical tape that is holding the membrane in

place and put the membrane in a 50-mL beaker. Cracks in the membrane are not

a concern at this point.

11. Add 5 mL of 6 M NaOH and let the membrane soak for 10 min. Swirl the

contents of the beaker occasionally. As the base dissolves the alumina, the

support ring will ﬂoat freely in the solution.

12. Remove the support ring from the solution using tweezers, rinse in water, and

discard. Black material will collect at the bottom of the beaker. Use tweezers or

a Pasteur pipette to break up any clumps of black material until the solution is a

cloudy gray color.

13. Set a magnet next to the side of the beaker and wait 1 min while the nickel

nanowires collect against the magnet. Use a Pasteur pipette to remove as much

of the alkaline solution as possible (without removing any of the black nanowire

material) and then add 15-mL deionized water to the beaker. Resuspend

the nanowires in the fresh water using a Pasteur pipette and collect the nano-

wires against a side again using a magnet. Remove the water again and rinse

once more.

14. After removin g water from the last washing, add 2–3 mL of ethylene glycol.

Other solvents can also be used, such as water or ethanol, but we have found that

the wires stay suspended longer in ethy lene glycol, allowing the students to

better see the response of their wires to magnetic ﬁelds using the optical

microscope. Transfer the suspe nsion to a small vial. Use a Pasteur pipette or

shake the vial to disperse the nanowires; the solution will appear cloudy and

gray. The nanowire suspension can be stored indeﬁnitely.

15. Transfer a drop of the nanowire suspension to a microscope slide and cover with

a coverslip. If clumps remain in the solution , try to avoid transferring them.

Even if the solution does not appear to be very cloudy, there are probably still a

large number of nanowires suspended, since each membrane has on the order of

one billion pores.

16. To examine the nanowires using SEM, place a drop of nanowires suspended

in water directly on an aluminum SEM specimen mount and allow the drop to

dry. (To obtain a smoother background in the images, attach a piece of silicon

708 Appendix C. Materials-Related Laboratory Experiments

to the specimen mount using carbon tape and place the drop of nanowire suspension

on the silicon.) No conductive coating needs to be added. We imaged the nanowires

using a LEO FE-SEM with a 3 kV accelerating voltage and 3 mm working distance.

17. To examine the nanowires by X-ray diffraction (XRD) while they are still

embedded in the membrane, tape the membrane to a glass microscope slide

with the Ni deposition side facing upward. To examine a sample of liberated

nanowires, suspend the nanowires in ethanol and place a few drops on a

microscope slide. After the solv ent evaporates, add a few more drops and repeat

this process until there is a visible pile of nanowires on the slide. Our patter ns

were obtained using a Scintag PADV powder X-ray diffractometer with Cu K-a

radiation, a tube voltage of 40 kV and a tube current of 35 mA.

C.5. INTRODUCTION TO PHOTOLITHOGRAPHY

(Berkowski, K. L.; Plunkett, K. N.; Yu, Q.; Moore, J. S. J. Chem. Ed., 2005)

C.5.1. Procedure

CAS Registry Numbers for Chemicals:

Isobornyl acrylate (IBA): 5888-33-5

Bisphenol-A-glycidyldimethyacrylate (Bis-GMA): 1565-94-2

2,2-dimethoxy-2-phenylacetophenone (DMPA): 24650-42-8

Oil Red O (Solvent Red 27): 1320-06-5

Oil Blue N (Sol vent Blue 14): 2646-15-3

Fluorescent Yellow 3G (Solvent Yellow 98): 12671-74-8

Ethanol: 64-17-5

To preve nt premature polymerization of the photoresist, store it in an amber vial

or a clear vial wrapped with aluminum foil in a chemical refrigerator, if available,

and minimize its exposure to light.

Photoresist preparation

1. Use a spatula to mass approximately 2.0 g of Bis-GMA into an amber vial. Bis-

GMA is extremely viscous and sticky and you may need an additional spatula to

push it to the bottom of the vial.

2. If some of the Bis-GMA sticks to the sides or top of the vial, tightly screw on the

vial lid and completely submerge the vial in a beaker full of hot tap water until

the Bis-GMA ﬂows to the bottom.

3. Add approximately 3.5 g of IBA to the vial containing Bis-GMA.

4. Add approximately 0.18 g (3% wt) of DMPA to the vial containing IBA and

Bis-GMA.

5. Sonicate the mixture for 30 min.

6. Swirl the solution to make sure it is homogeneous. If all components are not

dissolved, sonicate the solution for an additional 10 min.

7. Repeat step 6 until the solution is homogeneous.

C.5. Introduction to Photolithography 709

8. Add the appropriate color dye (1–1.5 mg or a small spatula tip) to the resulting

photoresist.

9. Sonicate this solution for 5–10 min.

10. Store the photoresist in an amber vial with a screw-top dropper in a chemical

refrigerator, if available. A clear vial wrapped with aluminum foil may also

be used.

Photomask preparation

1. Create a 3  3 in. black box in Microsoft Word to use as a template.

2. Copy and paste six of these black boxes on a single page to create multiple

photomasks.

3. Text-contain ing photomasks: create a text box, format it for white font and no ﬁll,

and center the text box inside the black box to allow at least a 0.5 in. black border

surrounding the text.

4. Picture-containing photomasks: manipulate shapes in Microsoft Paint by pasting

a white shape on a black background and copy this ﬁgure to the center of the

black box on your template.

5. Print the photomasks on transparency paper using a black and white printer at

a resolution of 600 dpi or higher.

Polymer patterning (Figure C.6)

1. Cut a transparency paper into four equal pieces of 5.5  4.25 in. and place one

of these ﬂat on the table.

2. Align two thin glass coverslips (22  50 mm) on the top and bottom of the

transparency.

3. Apply 15–30 drops of the photoresist to the center of the transparency leaving

about 2.5 in. between each coverslip and the liquid photoresist.

4. Cover the photoresist with a thick glass slide (75  50  1 mm) by placing the

slide in the center of the transparency and allowing its top and bottom to rest on

top of the thin coverslips.

5. Wait until the entire space between the thick glass slide and the transparency is

ﬁlled with photoresist.

6. Place the photomask on top of the thick glass slide and make sure that photore-

sist is visible through all parts of the photomask.

7. Place a second thick glass slide on top of the photomask to keep it in place.

8. Polymerize the exposed photoresist by shining UV light (365 nm) from the

hand-held lamp 0.5 in. above the sample for 20 s (time may be dependent on

the UV lamp).

9. Remove the photomask and carefully peel away the bottom transparency from

the thick glass slide.

10. Wash away the unpolymerized photoresist into a beaker with a wash bottle

containing ethanol.

11. Dry the resulting polymer by gently dabbing it with a paper towel.

710 Appendix C. Materials-Related Laboratory Experiments

Troubleshooting

1. Photoresist solution is not homogeneous – increase sonication time.

2. Photoresist solution polymerizes in container – store in an amber container or a

container wrapped with aluminum foil in a chemical refrigerator, if available.

3. Photoresist does not ﬂow to edges of glass slide or polymer pattern is too

thin – increase the initial amount of prepolymer solution on the transparency

ﬁlm.

4. Bubbles form in the polymer product:

(a) Poke away all bubbles with a pipette tip when photoresist is dropped on the

transparency.

(b) Carefully plac e the glass slide on top of the photoresist.

5. Incomplete or no polymerization upon exposure to UV light:

(a) Increase UV exposure time.

(b) Increase crosslinker concentration (Bis-GMA).

(d) Decrease amount of dye.

6. Brittle or delaminating polymer structures:

(a) Decrease UV exposure time.

(b) Decrease crosslinker concentration (Bis-GMA).

7. Polymer forms in areas that were not exposed to UV light:

(a) Decrease UV exposure time.

(b) Increase space between structures and letters on photomask.

transmission.

Figure C.6. Overview of the fabrication process. Two coverslips are placed at the top and bottom of a

transparency (a) and prepolymer solution is dropped in the center (b). A thick glass slide is positioned on

top of the prepolymer solution, allowing it to rest on the coverslips, and causing the prepolymer solution to

ﬂow and ﬁll the gap (c). A photomask is aligned on top of the glass slide (d), a second glass slide is placed

on top of the photomask to keep it in place, and the exposed photoresist is polymerized using UV light.

After rinsing, an insoluble, patterned polymer results (e).

C.5. Introduction to Photolithography 711

C.6. SYNTHESIS OF GOLD NANOCLUSTERS

(Adapted from Weare, W. W.; Reed, S. M.; Warner, M. G.;

Hutchison, J. E. J. Am. Chem. Soc., 2000, 122, 12890)

Gold nanoclusters are primarily used to catalyze the growth of nanowires (via VLS

or SLS routes), and are also utilized for biomedical applications such as targeted

drug delivery agents. As discussed in Chapter 6, a number of techniques may be used

to synthesize metallic nanoclusters. In this experiment , a gold precursor is trans-

formed into nanoclusters with diameters < 2 nm via a two-step reaction. The ﬁrst

step consists of phase-transfer into an organic layer where it reacts with triphenyl-

phosphine. In the second step, the mixture is reduced by sodium borohydride.

C.6.1. Procedure

CAS Registry Numbers for Chemicals:

HAuCl

·3H

O: 16961-25-4

Tetraoctylammonium bromide: 14866-33-2

Triphenylphosphine: 603-35-0

NaBH

: 16940-66-2

Toluene: 108-88-3

Hexanes: 110-54-3

Methanol: 67-56-1

Chloroform: 67-66-3

Pentane: 109-66-0

NaNO

, saturated solution: 7632-00-0

1. Dissolve hydrogen tetrachloroaurate trihydrate (1.00 g, 2.54 mmol) and tetra-

octylammonium bromide (1.60 g, 2.93 mmol) in a degassed water/toluene

mixture (50 mL/65 mL).

2. When the golden color is transferred into the organic phase, add triphenylpho-

sphine (2.32 g, 8.85 mmol) with vigorous stirring for ca. 10 min until the organic

phase is white and cloudy.

3. Add aqueous sodium borohydride (1.41 g, 37.3 mmol, dissolved in 10 mL of water

immediately prior to use) to the organic phase. The organic phase will immediately

turn dark purple. Continue to stir the solution for 3 h under a nitrogen ﬂow.

4. Separate the toluene layer, washing it with water (2  100 mL). Remove the

solvent in vacuo, or with a stream of nitrogen to yield a black solid.

5. Wash the resulting solid with a series of solvents (hexanes, saturated aqueous

sodium nitrite, and a 2:3 methanol:water mixture) to remove the phase-transfer

catalyst, byproducts, and unreacted starting materials.

6. Dissolve the solid in chloroform, and slowly add pentane to remove salts such as

AuCl(PPh

). Perform this precipitation procedure three times; after puri ﬁcation,

the yield is ca. 170 mg of puriﬁed nanoparticle from 1 g of HAuCl

712 Appendix C. Materials-Related Laboratory Experiments

7. To prepare the sample for TEM analysis, put a drop of the chloroform suspension

on a TEM grid and allow the solvent to evaporate. Alternatively, you may spray

the solvent suspension onto a grid usin g an aerosol delivery device.

Nice Papers (One Thorough Review Article) Related to Nanocluster Growth

1. Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G.

J. Am. Chem. Soc., 2004, 126, 273.

2. Cushing, B. L. ; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev., 2004, 104,

3893.

C.7. SYNTHESIS OF PORO US SILICON

Lopez, V.;

Badilla, J. P.;

Ramirez-Porras, A.;

Fahlman, B. D.

Centro de Investigacio

n en Ciencia e Ingenierı

a de Materiales (CICIMA),

Universidad de Costa Rica, San Jose, CR

Department of Chemistry, Central Michigan University, Mount Pleasant,

MI 48859

As you may recall from Chapter 4, bulk Si is an indirect bandgap semiconductor,

which limits applications that require radiative emission. However, when bulk Si is

electrochemically etched under controlled conditions, a nanoporous material known

as porous silicon (PS) is generated, which displays a pronounced photoluminescence

due to the quantum conﬁnement of photoexcited carriers.

[1]

Consequently, PS is of

great interest for a plethora of applications related to optoelectronics (advanced

displays), sensors, biosensors, and photovoltaics. However, the widespread use of

PS for applications has been limited due to surface oxidation that readily occurs

within minutes of its formation. A number of strategies may be used to passivate the

H-terminated Si surface by the covalent attachment of chemical species in order to

prevent oxidation.

[2]

Herein, we describe the formation of PS via electrochemical

etching in a HF solution. Though not described here, a great deal of experimental

variables may be varied during electrochemical etching (Table C.1), which may be

used to control the resultant morphology of the PS for the desired applications.

Table C.1. Effect of Varying Experimental Parameters on the Formation of Porous Silicon

Experimental parameter

Resultant porosity Resultant etching rate Resulting critical current

HF concentration Decrease Decrease Increase

Current density Increase Increase No effect

Anodization time Increase Minimal effect No effect

Temperature No effect No effect Increase

Wafer doping (p-type) Decrease Increase Increase

Wafer doping (n-type) Increase Increase No effect

Corresponds to an increase in each experimental parameter.

C.7. Synthesis of Porous Silicon 713

C.7.1. Procedure

CAS Registry Numbers for Chemicals:

Hydroﬂuoric acid, concentrated (48%): 7664-39-3

Ethanol: 64-17-5

Figure C.7 illustrates the steps required to electrochemically etch Si. A Teﬂon cell

ﬁtted with a Viton o-ring is shown in (a). A silicon wafer is placed polished side

down onto the top of the o-ring, (b). A piece of aluminum foil is placed on top of

the wafer backside and the plastic backplate is screwe d into place, (c). The polished

side of the wafer is shown from the top of the Teﬂon cell in photograph (d). The

wafer surface is treated with 10% HF(aq) for 10 min. to remove any native oxide

layer, followed by rinsing with water and ethanol. The cell is then ﬁlled with an

electrolyte consisting of 12.5% HF (HF:H

O:EtOH of 1:4:3), and a platinum

electrode is immersed into the solution, (e). Electrical connections to the platinum

and aluminum electrode surfaces are made (note: current ﬂows from the bottom to

top) and appropriate current started. Photograph (g) shows the presence of tiny

bubbles that indicate the electrochemical anodization of the silicon substrate. The

ﬁnal etched wafer is shown in (h), which is rinsed with water and ethanol and dried

under a ﬂow of nitrogen. Using a current of 54 mA.cm

2

for 20 min. for a p-type

Si(100) substrate with a resist ivity of 20–50 O.cm results in a macroporosity (70%)

with pores 2–3 mm in diameter and 40–50 mm depth (e.g., Figure C.8).

C.8. SOLID-LIQUID- SOLID (SLS) GROWTH OF SILICON NANOWIRES

Antic, A.;

Oshel, P.;

Fahlman, B. D.

Department of Chemistry, Central Michigan University, Mt. Pleasant,

MI 48859

Department of Biology, Central Michigan University, Mt. Pleasant, MI 48859

Nanowires are crystalline wires with characteristic diameters of less than 100 nan-

ometers (nm) and comparatively long lengths. Silicon nanowire arrays are of parti-

cular interest to progress current microelectronic technology into nanoelectronic

systems. The optical and electrical properties of nanowires are based on their

morphologies (i.e., composition, crystal structure, and growth orientation), which

are primarily based on the production method. Other semiconductors, such as gallium

arsenide (GaAs), are also of interest for use in light emitting diodes (LEDs) and lasers.

The most common method used to grow silicon nanowires is by vapor-liquid-

solid (VLS), which uses chemical vapor deposition onto a noble metal catalyst such

as gold nanoparticles from a gaseous precursor such as SiH

[3]

Herein, we describe

the growth of silicon nanowires (SiNWs) using solid- liquid-solid (SoLS

[4]

) growth

from gold nanoparticulate catalysts. To our knowledge, this is the ﬁrst report of

SoLS being used with nanoparticulate Au catalysts; the other sparse prec edents

[5]

using this technique have used metallic thin ﬁlms as the catalytic surface. It should

714 Appendix C. Materials-Related Laboratory Experiments

Figure C.7. Photographs of the experimental setup used to electrochemically etch a porous silicon region

on the surface of a single-crystalline Si wafer (steps (a) - (h), as described in the procedure).

C.8. Solid-Liquid-Solid (SLS) Growth of Silicon Nanowires 715

be noted that one should also be able to grow SiNWs from other face-centered

cubic (FCC) metals; different heating/cooling regimes will be required to determine

the most effective size-c orrected eutectic for those compositions.

C.8.1. Procedure

CAS Registry Numbers for Chemicals:

HAuCl

·3H

O: 16961-25-4

Sodium citrate: 6132-04-3

Hydroﬂuoric acid, 48 wt%: 7664-39-3

Ethanol: 64-17-5

Figure C.8. Scanning electron micrographs of a representative porous silicon ﬁlm etched onto p-type

Si(100) using the conditions described herein.

716 Appendix C. Materials-Related Laboratory Experiments

1. Prepare a nanoparticulate gold solution, using the Turkevich method as follows:

(a) Prepare 500 mL of a 1.0 mM stock solution of HAuCl

using ultrapure

(18 MO) water. Note: the HAuCl

.xH

O salt is very hygros copic and must

be stored in a dessicator before/after its use.

(b) Prepare a 1% (w/v) stock solution of sodium citrate using ultrapure (18 MO)

water.

stirring, heat the solution to boiling.

(d) Add 2 mL of stock solution b) to the boiling solution. Gold nanoparticles will

form as the citrate reduces the Au(III). Remove the solution from heat when

the solution has turned red.

2. Remove the native SiO

layer from electronic-grade crystalline Si wafer (111) or

(100) by immersing in a 1:10 HF:EtOH solution for 30 sec., followed by rinsing

with DI water. Dry with a KimWipe

3. Place two to three drops of the Au solution prepared in 1. above onto the wafer

via pipette or aerosol spraying, followed by drying by “ﬂash” heating with a hot

air gun, toggling the heat on and off and passing the airﬂow back and forth across

the sample.

4. Place the wafer in a ceramic boat inside a horizontal quartz tube within a tube

furnace, pre-ﬂushed with Argon for at least 20 min. Re-seal the connections and

continue to ﬂush with Argon for at least 30 min. Ramp the temperature of the tube

furnace to 1,000



C, and maintain this temperature for at least 15 min.

5. Cool the samples to room temperature within the tube furnace under a ﬂow of

Argon. Store samples in an inert-atmosphere glovebox (or dessicator) until

characterization measurements are performed. A high yield of silicon nanowires

should be evident from scanning electron microscopy (SEM); e.g., Figure C.9.

C.9. SYNTHESIS OF FERROFLUIDS

– Some online laboratory modules:

(a) http://mrsec.wisc.edu/Edetc/nanolab/ffexp/index.html

(b) http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm

C.10. METALLURGY/PHASE TRANSFORMATIONS

– Some online laboratory modules:

(a) http://dmseg5.case.edu/classes/EMSE-290-S11/Pages/instructions/Exp1.pdf

(b) http://www.csun.edu/~bavarian/mse_528.htm

C.11. HEAT TREATMENT OF GLASS CERAMICS

– Published online: http://neon.mems.cmu.edu/rollett/27302/27302.Lab1.glass.

ceramic.pdf

C.11. Heat Treatment of Glass Ceramics 717