Unconventional Patterning at the Nanoscale
NanoEd Resources - Nano Lessons and Courses
Written by Prof. Teri W. Odom, Dr. M. Viswanathan and Y. Babayan   
Wednesday, 07 February 2007 10:35
Authors:
Prof. Teri W. Odom,
Dr. M. Viswanathan and Y. Babayan

Institution:
Northwestern University
Evanston, IL USA

Level:
College and above

In the classroom:


This Course is a video lab manual for hands on fabrication and characterization of materials at the nanoscale.

Materials requirements range from simple chemicals, benchtop tools and CDs to necessary access to advanced characterization equipment such as an Scanning Tunneling Microscope.

INTRODUCTION

The ability to generate nanoscale structures is central to modern science and technology. The experiments described in this site provide the first step towards bringing nanofabrication to undergraduate students using simple benchtop tools that are accessible and inexpensive. These experiments are currently taking place in the first quarter of a sophomore seminar (Chem 250-1) offered jointly by the Weinberg College of Arts and Sciences and the McCormick School of Engineering at Northwestern University.

This webpage provides a step-by-step video demonstration of various nanoscale patterning experiments and explains the basic principles behind the techniques. Students learn (i) how to pattern nanometer-sized features using soft lithography (microcontact printing, molding, and phase-shifting photolithography), (ii) how to synthesize and characterize nanoparticles and nanoscale devices, and (iii) how to create simple nanoscale optical devices. Students also receive training on nanoscale characterization tools, such as scanning electron microscopy and atomic force microscopy.

The design of these experiments and video modules was made possible by the National Science Foundation under the Nanotechnology in Undergraduate Education Award, DMR-0407073.

OBJECTIVES:

This video lab manual is the first part of a two-sequence, research-based course on nanoscale patterning for undergraduate students offered by Prof. Teri W. Odom and Prof. Vinayak P. Dravid. The video lab manual has a set of nanopatterning experiments in addition to few nanoscale syntheses as well as nanoscale devices. These experiments enable undergraduate institutions that want access to techniques in nanopatterning but do not have the facilities or resources. The projects can be performed on the benchtop with a minimal arsenal of supplies. The experiments use only accesible materials, such as UV lamp, light sensitive polymers, CD / DVDs and some chemicals. Students have the opportunity to get hands-on training and experience with nanopatterning using soft lithography. There is a step-by-step movie demonstration of the experiments with educational modules for many experiments. In addition, there are also tutorials that explain the basic principles behind each of the experiments.

The courses at Northwestern (Chemistry 250-1 and 250-2), offered Winter and Spring 2006, provide research experience on top-down fabrication and bottom-up synthesis, including interactive training in nanoscale characterization tools. The classes are designed to expose students early in their academic careers to new concepts in nanotechnology using interdisciplinary ideas in chemistry, materials science and engineering. Because nanotechnolgy is such a broad subject area, we have focussed on a particular aspect of nanotechnology — nanoscale patterning and characterization -- to maximize hands-on experience of students in research projects.

Specific course objectives include:

  • Identification of current and unconventional methods in nanoscale patterning
  • Training on fabrication of nm and micron scale structure
  • Training on advanced equiment (scanning probe and electron microscopes)
  • Participation in group research projects
  • Designing independent research proposals and projects
  • Preparation of students for more advanced courses in nanotechnolgy
  • Development of effective research-based courses that promote student intellect

NANOSCALE TOOLS

Fabrication of Masters

Fabrication of Master

Conventionally, masters are fabricated using photolithography. Here we use inexpensive commercially available CD / DVDs as masters. These masters will be used later to prepare PDMS stamps.

MATERIALS:

  • Sony D-R CD / DVD
  • Sharp scissors
  • Tweezers
  • Petri dish

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
 Download Player: For Mac | For PC


Cut the CD (the one used is Sony CD-R) with scissors. There is a polycarbonate (PC) surface underneath and a peelable Al layer. Some of them also have an acrylic layer on the top of Al layer and a label as the top surface.

master2


Peel the Al layer off the PC base using tweezers. Save both the Al layer and the PC surface for AFM imaging, placing both patterned side face-up in the petri dish. You can use the PC surface or the Al layer as the master for soft lithography.


Preparation of PDM Stamps

Preparation of PDM Stamps

(PDMS - POLY DIMETHYLSILOXANE

An elastomeric stamp is the key element of soft lithography. It is usually prepared by replica molding, by casting the liquid pre-polymer of an elastomer over a master that has patterned relief structures on its surface.

WHY USE PDMS?

It is the unique surface behavior of PDMS that allows for most of the applications. The surface energy is a manifestation of intermolecular forces. The organic portion in PDMS is the methyl group, which has almost the weakest intermolecular forces. The inorganic siloxane backbone, one of the most flexible polymer backbone, allows the methyl groups to be arranged such that PDMS has almost the lowest surface energies known (expensive fluorocarbon polymers have the least surface energy).

MATERIALS:

  • » Sylgard 184 elastomer (consists of pre-polymer and curing agent)
  • » Dessicator
  • » An oven capable of reaching 70°C
  • » Master that has patterned relief structures (CD / DVD)
  • » Plastic cup, Plastic fork, Sharp scalpel, petri dish

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
 Download Player: For Mac | For PC

movie1-pdms

In a plastic cup, weigh 10 parts (by weight, ~ 20 g) of Sylgard 184 pre-polymer and 1 part (~ 2g) of curing agent.

 

movie2-pdms

Mix the above vigorously for a couple of minutes until the entire mixture is filled with bubbles.

 

movie3-pdms

Place the cup in a dessicator to degas (allow bubbles to rise out) for 20 minutes.

 

movie4-pdms

Pour the mixture slowly into a petri dish that has prepatterned structure (DVD/CD). Avoid bubbles and make sure that the master is lying flat on the bottom of the petri dish. It should cover the CD/ DVD masters completely. The PDMS layer should be about 2 mm (you may not use all of the polymer).

 

movie5-pdms

Place the petri dish in an oven and cure it at 70°C for an hour.

 

pdms-scalp

Using a sharp scalpel, evenly and gently cut around the pattern.

 

pdms-cut

Remove the stamp using a tweezer. The movie depicts a small stamp being cut from the much larger polycarbonate master of the CD.

 

Cleaving Si Wafers

Cleaving Si Wafers

One of the most essential nanoscale skills is cleaving Si wafers. This skill with be practiced in this lab exercise producing substrates that will be used in future patterning labs.

MATERIALS:

  • » Si (100) Wafers
  • » Wafer tweezer
  • » Diamond Scribe, petri dish

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
 Download Player: For Mac | For PC

Press firmly down on the flat edge (indicates orientation along (110)) of the Si wafer using a diamond scribe while lightly holding a wafer tweezer on the opposite side. The wafer should break nicely in a straight line in half.

 

Follow the same procedure as above to cut each of the above halves into two.

 

Cut the Si wafer into eight pieces. Carefully place these pieces in a labelled petri dish for storage. These pieces will be used in future patterning labs.

More about PDMS

Preparation of PDMS Stamps - More about PDMS

Polydimethylsiloxane (PDMS) can be produced in the form of a colorless, transparent elastomer that can be used for a variety of applications. The use of this elastomer for replicating micro / nano structure has been demonstrated by Whitesides et al. PDMS has a unique combination of properties resulting from the presence of inorganic siloxane backbone and organic methyl groups attached to silicon. They are fluids at room temperature as they have a low glass transition temperature. They can be converted to solid elastomers easily by cross linking. The formulation, fabrication and applications of PDMS elastomers has been well documented in literature.

PDMS is marketted as a kit that consists of a base and a curing agent. (Ref: MRSEC PDMS article)

Several properties of PDMS are instrumental in the formation of high-quality patterns and structures in soft lithography.

  • » PDMS is an elastomer and conforms to the surface of the substrate over a relatively large area.
  • » PDMS is deformable enough such that conformable contact can even be achieved on surfaces that are non-planar on a micrometer scale.
  • » Elastic characteristics of PDMS allows it to be released easily from complex and fragile structures.
  • » PDMS provides a surface that is low in interfacial free energy (21.6 x 10 -3 Jm-2) and chemically inert.
  • » PDMS is homogeneous, isotropic and optically transparent down to 300 nm.
  • » PDMS is durable and the stamps can be used upto about 100 times over a period of several months without noticeable degradation.
  • » Suface properties of PDMS can be modified by plasma treatment followed by the formation of SAMs (self assembled monolayers) to give appropriate interfacial interactions with materials that themselves have a wide range of interfacial free energies.

The low elastic modulus for sylgard 184 PDMS makes it difficult to replicate nanoscale features with high aspect ratios using soft lithography.

CHARACTERIZATION

Atomic Force Microscopy

Atomic Force Microscope (AFM)

The atomic force microscope (AFM) or scanning force microscope (SFM) was invented in 1986 by Binnig, Quate and Gerber. Similar to other scanning probe microscopes, the AFM raster scans a sharp probe over the surface of a sample and measures the changes in force between the probe tip and the sample. A cantilever with a sharp tip is positioned above a surface. Depending on this separation distance, long range or short range forces will dominate the interaction. This force is measured by the bending of the cantilever by an optical lever technique: a laser beam is focused on the back of a cantilever and reflected into a photodetector. Small forces between the tip and sample will cause less deflection than large forces. By raster-scanning the tip across the surface and recording the change in force as a function of position, a map of surface topography and other properties can be generated. You will use the AFM to image the CD/DVD masters and PDMS stamps and compare these images to the information provided by scanning electron microscopy.

Modes of Operation for the AFM:

There are three general types of AFM imaging: (1) contact mode, (2) tapping mode and (3) non-contact mode. In this lab, contact mode and tapping mode will be used to image the masters and PDMS stamps you made in the first lab.

Contact mode is the most common method of operation of the AFM and is useful for obtaining 3D to pographical information on nanostructures and surfaces. As the name suggests, the tip and sample remain in close contact as the scanning proceeds. One of the drawbacks of the tip remaining in contact with the sample is that large lateral forces can be exerted on the sample as the tip is dragged over the specimen. These large forces can result in deformed images and damaged sample.

Tapping mode is another mode of operation for AFM. Unlike the operation of contact mode, where the tip is in constant contact with the surface, in tapping mode the tip makes intermittent contact with the surface. As the tip is scanned over the surface, the cantilever is driven at its resonant frequency (hundreds of kHz). Because the contact time is a small fraction of its oscillation period, the forces that can deform the images or damage the sample are reduced dramatically. Tapping mode is usually preferred to image samples with structures that are weakly bound to the surface or samples that ares soft (polymers, thin films).

MATERIALS:

  • » AFM
  • » PM tips and Mount
  • » Polycarbonate and Al Masters from CD/DVD
  • » PDMS Stamps from CD/DVD Masters
  • » SAM samples for LFM

PROCEDURE:

Laboratory procedures

You have an opportunity to use a state-of-the-art AFM instrument from JEOL to characterize the samples you will pattern at the end of the course. You will use (1) tapping mode to image the CDs and DVDs and (2) contact mode (lateral force microscopy) to image microcontact printed patterns of self-assembled monolayers (SAMs) on gold.

Samples can be mounted to the magnetic sample holder (puck) using double sided tape. The samples are loaded on the scanner of the JEOL SPM 5200, whose maximum scan size is 65 x 65 m m 2 , and whose vertical range is ~5 m m. You will practice mounting tips into the tip holder using tweezers. Once the sample is loaded and the tip is placed above the sample, you need to align the laser beam onto the cantilever. You can get a rough idea where the beam is using the optical microscope situated above the AFM. Once the signal is maximized, you can bring the tip close to the surface and start the engage process. Specific operational details can be found in the training manual designed by Dr. Gajendra Shekhawat at the following link.

Samples and their preparation

  1. SAMs on gold surfaces by microcontact printing. You will be provided samples made by microcontact printing.
  2. CDs and DVDs. In the basics lab, you cut CDs or DVDs into small pieces to use as cheap masters. You will use tapping mode to image these periodic structures as well as your PDMS stamps -- and will eventually compare these patterns with ones you will make later in the course.

Questions for Lab Write-up:

Tip Characterization

  1. What types of tips did you use for these experiments?
  2. Why are some tips more suited for a particular operating mode?
  3. Describe how the tip geometry could affect the image resolution and induce artifacts in the image.

Sample Characterization

  1. Compare the tapping mode images of your PDMS stamp and your CD master.
  2. Comment on the difference between the topographic and lateral force images of the SAMs patterned on gold substrates.
  3. How useful was the lateral force imaging mode? Was it difficult to obtain an image with good chemical contrast?

» More about AFM (PDF)

Dip Pen Nanolithpgraphy

Dip Pen Nanolithography (DPN)

In a previous lab, Atomic Force Microscopy (AFM) was utilized to image the micron sized features of DVD's and/or CD's and the microcontact printed 2 micron lines of SAM's on a gold substrate. In this lab, an AFM tip will be used to write alkanethiol SAM patterns into gold substrates. You will practice writing a variety of shapes onto the surface and then image the pattern with Lateral Force Microscopy (LFM).

Modes of Operation for the DPN:

The three general types of DPN imaging are similar to those for AFM imaging: (1) contact mode, (2) tapping mode and (3) lateral force mode. Lateral force imaging-rather than topographical imaging-is generally the best way to scan completed DPN patterns because the system deposits ink in thin layers (as thin as one molecule) that would be hard to find topographically. LFM operates well under fast scan conditions. Quick imaging with the inky pen minimizes spurious ink deposition.

MATERIALS:

  1. DPN (PDF)
  2. SPM tips and Mount
  3. Alkane Thiol Solution
  4. Gold Substrate

PROCEDURE:

Using the skills you acquired in the previous lab, mount the gold substrate using double-sided tape and mount the thiol-covered probe tip. Adjust the AFM as instructed.

You will pattern dots (30-200 nm in diameter), lines, and other shapes of self-assembled monolayers (SAMs) on gold-coated silicon surfaces using DPN. Image your patterns by LFM.

QUESTIONS:

Tip Characterization

  1. What type of cantilever did you use for DPN experiments and why?
  2. Why are diving board cantilevers preferred over A shape cantilevers in DPN?
  3. What sized patterns did you generate using DPN nanolithography?
  4. Name four factors which determine the size of the nanopatterns during DPN writing?
  5. Why is LFM mode imaging preferred in DPN over topography?

» More about PDN (PDF)

Scanning Electron Microspcopy

Scanning Electron Microscopy (SEM)

INTRODUCTION:

This laboratory is designed to introduce the Hitachi S-3500N Scanning Electron Microscope. You will investigate methods for sample preparation and look at the effects of condenser lens strength. In this initial laboratory, the methods for basic operation and alignment of the instrument will be covered. The purpose of this lab is to get you familiar with the standard operation of the SEM and general facility practices.

As you know from the class, SEM merely provides the means to generate and analyze myriad signals generated from electron-specimen interactions. Of all these signals, the most prolific imaging signal is from secondary electrons (SEs). Much of the imaging in this and many other labs will be done by imaging the SE signal in SEM.

OBJECTIVE:

By the end of this laboratory session, you should be able to:

  1. Prepare and mount both conductive and insulating samples for examination.
  2. Start and align the Hitachi S-3500N SEM and explain the effects of the alignment upon the imaging conditions.
  3. Increase the resolution of the SEM image through the use of the condenser lenses and explain the optics behind this increase.

Since SEM imaging involves bombarding a material with an electron beam, the surface of the sample will accumulate charge if the electrons are not allowed to escape from the surface via a conductive path. If there is no such path, the image formed by the SEM will be very poor. Charging can also lead to excessive heating of the sample, causing material degradation. Insulating and semiconducting materials, i.e. ceramics, polymers, and organics, should be coated with a conductive material to prevent surface charging. Usually specimens are coated with either a metal or carbon.

For this laboratory you will be provided with samples of anodic aluminum oxide. One of the samples is uncoated and one sputter-coated with a platinum/palladium alloy. Since aluminum oxide is electrically insulating, you should be able to see artifacts from the sample charging with the uncoated sample.

MATERIALS:

  1. PDMS stamps and Masters
  2. Au nanoparticles
  3. ZnO nanorods on silicon
  4. e BL patterned silicon
  5. Hitachi 3500 SEM

SPECIMEN PREPARATION: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
  Download Player: For Mac | For PC

Since SEM imaging involves bombarding a material with an electron beam, the surface of the sample will accumulate charge if the electrons are not allowed to escape from the surface via a conductive path. If there is no such path, the image formed by the SEM will be very poor. Charging can also lead to excessive heating of the sample, causing material degradation. Insulating and semiconducting materials, i.e. ceramics, polymers, and organics, should be coated with a conductive material to prevent surface charging. Usually specimens are coated with either a metal or carbon.

Note: SEM's are vacuum instruments. One should always wear gloves when handling pieces that go inside the microscope vacuum chamber!

Assembly of sample holder

 

SEM-sample2 Mounting the sample
SEM-sample3

Fastening the sample

 

SEM-sample4

Checking for the height!

 

SEM-sample5

Adjusting the height

 

STARTING UP THE MOCROSCOP:

Even though SEM's are expensive and technically advanced analytical tools, don't let them be intimidating. For the SEM on which you will be working, there are only a few ways in which you can damage the scope. If you remember the following things you will be fine:

Always wear gloves when handling your sample and the SEM sample holders. This helps keep the vacuum system clean and avoids sample contamination.

Never vent the chamber without shutting off the high voltage! Doing this can cause the tungsten filament to rapidly oxidize and burn out.

When changing the accelerating voltage, follow the proper procedure and saturate the filament. To change the accelerating voltage:

  1. Shut of the high voltage
  2. Change the voltage level
  3. Turn the voltage back on
  4. Re- saturate the filament.

Make sure that the z-height is adjusted correctly and the BSE detector is withdrawn when loading and unloading the sample. If the sample height and stage positions are not correct, you run the risk of hitting the objective lens or BSEdetector. A height gauge is available to ensure that your sample is not too tall when the z-height is set to the position for loading and unloading.

Starting the SEM consists of applying an electrical potential across the electron gun and running current through the filament. The current causes the filament to heat up, and once enough energy is supplied, electrons are ejected from the filament towards the anode. The bias on the Wehnelt cylinder can be adjusted to control the amount of electrons leaving the electron gun assembly. The S-3500 software does this process automatically, but it is still important to understand what is going on in the electron gun. For operation instructions, we will refer to the S-3500 manual for standard operating procedures.

ALIGNING THE MICROSCPE COLUMN:

In order to obtain good resolution it is essential that the microscope be properly aligned. Ideal alignment is achieved when the gun, lenses, and apertures are concentric about the optic axis - an imaginary line drawn down the center of the column. Before aligning, try to get your sample in the best possible focus, and capture a digital image of your sample. Use these conditions: 20,000X magnification, ~15mm working distance, 25kV accelerating voltage, objective aperture #3 and condenser lens strength (called beam current in software) set to 50.

Align the objective aperture:

When aperture alignment in the Operation/Alignment menu is selected, the focus wobble is activated. The focus wobble automatically changes the focus of the objective lens and aperture misalignment is manifested in translation of the image. To correct the aperture misalignment, adjust the X and Y knobs on the aperture to stop the image movement. (Note: depending on the initial gun alignment, it may be necessary to repeat this step after step 2 below) Align the electron gun: The gun alignment in the S-3500 is automatic – simply select Gun Shift, press the AGA (Auto Gun Align) button, then select Gun Tilt and press AGA again. We can, however, adjust the Gun Shift and Tilt manually with the X and Y controls and look for the brightest image on the screen. Try both and see how good the automatic alignment really is.

Align the stigmators:

In order to adequately correct for astigmatism, it is important that the stigmators are aligned along the optical axis. For both the X and Y stigmator alignments, use the X and Y controls in the software to stop the image movement (similar to aperture alignment).

Correct for astigmatism:

Once the stigmators have been aligned, you can adjust the strength of each stigmator independently to correct for astigmatism. You are looking for the sharpest image and should see no stretching of the image when changing focus.

Once the microscope has been aligned, take another picture of your sample in the same region and at the same magnification.

CONTROLLING RESOLUTION WITH THE CONDENSER LENS:

Since the SEM uses an electron probe to scan in the image of our sample point-by-point, the diameter of the electron probe contributes to the resolution of that image. That is, a smaller electron beam diameter will create an image with higher resolution. One way to control the diameter of the electron beam, or probe size, is by manipulating the condenser lenses.

The purpose of the condenser lenses is to demagnify the probe coming from the gun assembly. The ultimate effect of the condenser lenses on the probe size is illustrated in the figure below (from Goldstein et al., 52):

From the diagram, you can see that for a given working distance and objective aperture size, a stronger condenser lens will produce a smaller electron probe size (read - higher resolution). But also notice that the stronger condenser lens setting results in a lower probe current, indicated by the larger crosshatched area (more electrons are being "thrown away" from the beam). In electron imaging, this loss of probe current will be seen as a picture with more "snow" or "noise". This is the dichotomy that you will encounter over and over in using electron probe instruments - spatial resolution versus analytical sensitivity.

» Flash tutorial on the effect of condenser lens in controlling resolution

LAB PROCEDURE:

Please refer to "More about SEM" for further details concerning this lab!!

Part 1:

You should capture two images of the Au nanoparticles sample - one before alignment and the other after alignment using the following conditions:

Sample: Au nanoparticles

  1. Accelerating Voltage: 25kV
  2. Beam Current: 30
  3. Detector: SE
  4. Objective Aperture: 2
  5. Working Distance: 10mm
  6. Magnification: >15,000X

Part 2: Signals in the SEM

The sample for this portion of the laboratory is a silicon wafer with features patterned with electron beam lithography ( e BL). This sample was created with two separate e BL steps in order to create features with two different metal films � pure Titanium and a 60/40 Au/Pd alloy. You should capture images of patterns with each type of metal using both the SE and BSE detectors.

SEM Conditions for Part 2:

Sample: e BL patterned silicon

  1. Accelerating Voltage: 25kV
  2. Beam Current: 50
  3. Detector: SE and BSE
  4. Objective Aperture: 2
  5. Working Distance: 15mm
  6. Magnification: ~10,000X

Part 3: Resolution vs. Signal to Noise

Tanstaafl. There ain't no such thing as a free lunch. This axiom is quite applicable to the problems of achieving high resolution images in the SEM. For this portion of the laboratory, we will concentrate on methods for improving imaging resolution in the SEM and the trade-offs involved. There are two major aspects to achieving high resolution images: the ability to form small probe of electrons and the ability to detect high resolution signals produced by the interaction with the sample.

The sample for this portion of the laboratory is ZnO nanorods on silicon . We will explore the effects of objective aperture size and condenser lens strength on resolution and signal to noise ratio. You will capture at least 4 images for this section comparing the at least two different aperture sizes and two different condenser lens strengths (beam currents).

Sample: ZnO nanorods on silicon

  1. Accelerating Voltage: 25kV
  2. Beam Current: 30 and 60
  3. Detector: SE
  4. Objective Aperture: 1 and 4
  5. Working Distance: 15mm
  6. Magnification: >20,000X

Part 4:

Image the PDMS masters from lab 1.

LAB QUESTIONS: Questions for laboratory write-up

  1. You may have noticed that to get a good BSE image, you need to have weak condenser lens settings and a large objective aperture. Why do you think this is the case?
  2. For the Au/Pd patterns on silicon, why do the smaller features appear darker than the larger ones if they have the same composition?
  3. Note that even though the image on the screen may get noisier with decreasing aperture size or increasing condenser lens strength, the photo taken may not show this. Please explain why this occurs.
  4. Since SEM is essentially a surface analysis technique, why doesn't sputter coating obscure useful surface information? Can you think of any cases where the coating might become a problem?
  5. Low voltage microscopy is a useful method for minimizing charging effects. What are some of the other benefits and tradeoffs? Why is cold field emission well suited for this technique?
  6. If you didn't have a dedicated BSE detector in your SEM, can you think of a way to use the E-T SE detector for this purpose?
  7. The image below is a pattern produced by electron beam lithography in PMMA on a silicon substrate. The PMMA was first spun onto the wafer (~150nm thick) and then the inner and outer lines of the letters in the �NUANCE' pattern were written by taking control of the SEM scan coils. After exposure to the electron beam, the PMMA becomes more soluble in the developer and can be removed. After developing, the sample was then sputter coated with a uniform thin film of Pt/Pd prior to imaging. For reference, the letters are ~2 m m tall and the lines are ~30nm wide. Why do the N, A and E have different contrast than the rest of the letters and the rest of the sample? Why are they dark? (This pattern was written several times on the sample and different letters were dark in each, so there is no difference in processing from letter to letter besides random variation .) Also, try to explain why the center of the 'A' is darker that the rest and why the 'E' is darker on the horizontal parts.

 

» More about SEM (PDF)

SOFT LITHOGRAPHY 

Molding-Replica Molding (RM)

Molding - Replica Molding (RM)

Replica molding and related molding techniques are practical methods for fabricating structures as small as 30 nm in organic polymers with accuracy in vertical dimension of 5 nm.

MATERIALS:

  1. Patterned PDMS mold
  2. Liquid Polyurethane prepolymer
  3. Glass Slides
  4. UV lamp
  5. Sharp scalpel

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
  Download Player: For Mac | For PC

Replica

Clean the glass slide with soap and water and then rinse with ethanol. Dry under a stream of nitrogen.

 

Replica2

Using a fresh blade on your scalpel, cut out a stamp that is patterned with circles or other shapes.

 

Replica3

Place several drops of the liquid polyurethane prepolymer onto a glass slide (so that it creates a puddle about the size of your stamp)

 

Replica4

Gently place the patterned side of the stamp into contact with the liquid polymer.

 

Replica5

Place your sample under a UV lamp for five minutes.

 

Replica6

Remove the stamp and place facedown on a clean glass slide. Look at your sample underneath an AFM.

 


QUESTIONS:

  1. Characterize your RM, MIMIC and SAMIM samples using the AFM and compare it to your PDMS mold. How well did the pattern transfer?
  2. Compare and contrast the three molding techniques—RM, MIMIC, and SAMIM. What are their advantages and disadvantages?
  3. Which technique produced the best replication of the CD pattern from the PDMS stamp?
  4. In molding techniques, how does the pattern compare with the pattern of the stamp?
  5. How could you obtain the exact pattern as the one on the stamp?

Molding-Micromolding in Capillaries (MIMIC)

Molding - Micromolding in Capillaries (MIMIC)

MIMIC is a simple, convenient method to fabricate three-dimensional micro/nano structures of polymers, ceramics, etc. The techinque is based on the spontaneous filling of capillaries formed between two surfaces in conformal contact, at least one of which has a recessed relief structure, by a fluid. The fluid may be a liquid prepolymer or suspension of materials to be formed.

MATERIALS:

  • Patterned PDMS mold
  • UV Lamp
  • Prepolymer Polyurethane
  • Sharp scalpel, Razor blade, Glass slides

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
  Download Player: For Mac | For PC

mimic1

Using a fresh blade on your scalpel, cut out a stamp that is patterned with lines. Determine which direction the lines are running.

 

mimic2

Clean four glass slides using soap and water, and then rinse them with ethanol. Dry under a stream of nitrogen.

 

mimic3

With a new razor blade, press down vertically on the ends of the stamps that are perpendicular to the direction the lines are running.

 

mimic4

Place the patterned side face down onto your clean glass slide.

 

mimic5

Place a drop of prepolymer Polyurethane at the open end and wait 10-15 minutes while the channels fill by capillary action.

 

mimic6

Place the samples 10 cm under a UV lamp and cure the polymer for 5 minutes. Remove the PDMS stamp

 

mimic7

Remove the PDMS stamp. You should be able to see the diffraction pattern on the surface. If you do not see the diffraction pattern, you may have to start over using a dessicator to help fill the capillaries. Image the glass slide under the AFM.

 


QUESTIONS:

» See the Replica Molding Section.

Molding-Solvent-Assisted Micromolding (SAMIM)

Molding - Solvent-Assisted Micromolding (SAMIM)

SAMIM is useful for fabricating structures and modifying surface morphologies of polymers. The operational principle of this technique shares characteristics with both embossing and replica molding.

PRINCIPLE:

The key element of SAMIM is wetting of the PDMS mold by a solvent and conformal contact between the elastomer mold and the substrate. The liquid solvent fills the recessed regions on the surface of the PDMS mold in order to minimize the area of the liquid/vapor interface and maximize that of the solid/liquid interface.

MATERIALS:

  • » Patterned PDMS mold
  • » Photoresist coated Si wafers
  • » Ethanol
  • » Sharp scalpel, Razor blade, Glass slides

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
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SAMIM1

Using a fresh blade on your scalpel, cut out a stamp that is patterned with lines.

 

SAMIM2

Cut the photoresist coated silicon wafer into 4-8 pieces. Remember, the photoresist is sensitive to light!!

 

SAMIM3

Wet a clean cloth with ethanol and rub the surface of the patterned stamp.

 

SAMIM4

Immediately place the stamp into contact with the photoresist coated surface.

 

SAMM5

Keep the stamp in conformal contact with the surface until the solvent evaporates (2-3 min). Remove the stamp from the surface.

 

SAMM6

You should see an even diffraction pattern indicating that you have embossed the surface of the photoresist.

 


QUESTIONS:

» See the Replica Molding Section.

Microcontact Printing and Etching

Microcontact Printing and Etching

This lab introduces the technique of patterning self-assembled monolayers (SAM) on gold using microcontact printing. The gold patterns are made using SAMs as etch resist.

MATERIALS:

  1. Patterned PDMS stamps
  2. Octadecanethiol, Ethanol
  3. Gold Etchant: from Trancene
  4. Cotton swabs, spatulas, glass vials, stir bars, beakers

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
 Download Player: For Mac | For PC

 Preparing Substrates for MicroContact Printing and PDMS Stamps
mcp1

Prepare a surface for microcontact printing and etching by first cutting a small piece from the gold CD (Au-CD) provided.

 

mcp2

Dissolve the protective coating on the surface of the Au-CD with nitric acid.

 

mcp3

Carefully pour nitric acid into waste container.

 

mcp4

Rinse the Au-CD piece with distilled water and dry with nitrogen. This process should leave the relief structure of the Au-CD exposed. This gold piece can now be used as a substrate (along with provided gold-coated silicon wafer pieces) for mCP and etching.

MCP6

Cut out a stamp from your CD master.

 


 MicroContact Printing on Au CDs and Au Surfaces
MCP8

Make a 1 mM solution of octadecanethiol in ethanol and shake it well. This solution is your "ink" solution.

 

MCP9

Take a cotton swab and dip it into the above solution. Rub it back and forth across the patterned side of the PDMS stamp for 10 s.

 

MCP10

Dry the ink under a stream of nitrogen until dry (approximately 1 min).

 

Hint (and aside):

To determine the direction of the lines on the PDMS mask shine a laserpointer through the substrate (at 90 deg) held above a piece of whitepaper. Based on the diffration pattern observed, you should be able to determine the direction of lines. This procedure can also be done for CDs after you etch the Au layer off (see etching section below). Note: this video will open in a separate window.

MCP11

Place the stamp in conformal contact with the gold surface for 5 s or less. Repeat this process with both the Au-CD substrate and the gold-coated silicon wafers provided. For the Au-CD substrate, place the stamp such that the lines of the PDMS stamp run perpendicular to the lines on the Au-CD (determine direction of lines on the mask and the Au-CD using the laser pointer method or previous knowledge). Try stamping the line patterns at different angles. Experiment also with the differently patterned PDMS stamps.

MCP12

Remove the stamp from the surface. To visualize the pattern, you can breath on your chip. You should see a diffraction pattern if you have transferred the pattern from the stamp to the gold surface. Save one good microcontact printed sample (one that has NOT been etched). Image (1) this sample and (2) those made on Au substrates (not CDs) using LATERAL FORCE MICROSCOPY (LFM).


  Etching using SAMs as a Resist Layer
MCP14

Dilute the commercial gold etchant with deionized water (1:3).

 

MCP15

Stir the etchant well with a stirbar.

 

MCP16

Using plastic scissor clips, place your SAM coated gold into the beaker containing the etch solution. Etch for approximately 38-41 seconds. You may have to adjust these times slightly for the best effect. Rinse off the substrate in a stream of water and dry under nitrogen. Look at the patterns on the Au-CD under the optical microscope. Image (1) micron-sized features on the Au substrates and (2) the Au patterns CDs UNDER THE AFM (and if possible, the SEM).

To visualize the Au cross hatch pattern after etching on a CD, you can shine a laser pointer onto the sample and look at the diffraction pattern that is produced. Note: this video will open in a separate window.

 


QUESTIONS:

  1. What were the best "stamping" times for the PDMS stamp from the CD-master and the PDMS stamp with micron-sized features?
  2. Compare the LFM images of the patterns with micron-sized and nanometer-sized features.
  3. What are the limitations of micro-contact printing? How does diffusion of the "ink" play a role?
  4. Compare the AFM images of the etched Au patterns with the LFM images.
  5. Is microcontact printing followed by etching a good way to make micro- or nanostructures? Why or why not?

NANOSCALE SYNTHESIS

Synthesis of CdSe Nanocrystals

Synthesis of CdSe nanocrystals

The objective of this experiment is to synthesize semiconducting CdSe nanocrystals in solution and observe their size-dependent properties.

BACKGROUND:

Nanocrystals are interesting because they exhibit unique properties depending on their size. For example, CdSe is a semiconducting material whose energy bandgap is in the visible region (~650 nm). When these particles are made very small (2-5 nm), quantum confinement produces a range of different colors depending on the size of the particle. The origin of this effect is because the size of the particle becomes smaller than the size of the charge carriers responsible for light emission. In this lab you will synthesize CdSe nanoparticles and characterize their optical absorption and emission spectra.

MATERIALS:

  1. CdO
  2. TDPA (Tetra decyl phosphonic acid)
  3. Selenium
  4. TOPO (Tri octyl phosphine oxide, high boiling point solvent)
  5. TOP (Tri octyl phosphine)
  6. Three neck round bottom flask, Thermometers, stir bars, glass vials, spatulas, pipettes, bulbs, hot plate, heating
  7. mantle, clamps, cuvettes

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
 Download Player: For Mac | For PC

CdSe2

Weigh 0.062 g of CdO (0.5 mmol) and 0.278 g of TDPA (1 mmol) to a 10 ml three neck round bottom flask. (Warning: CdO is an inhalation hazard and this operation should be done in a fumehood)

 

CdSe3

To the same flask, add 3.678 g (~10 mmol) TOPO.

 

CdSe4

Insert a thermometer in one neck and add a magnetic stir bar in the flask.

 

CdSe5

Close the other two necks with a rubber septa.

 

CdSe6

Clamp the flask in a heating mantle.

 

CdSe7

Heat the solution to 320 degrees C to dissolve the CdO.

 

CdSe8

If you have access, plug in a nitrogen inlet.

 

CdSe9

Use a syringe needle as an outlet. Otherwise you can skip this and the previous step.

 

CdSe10

As you wait for CdO to dissolve, add 0.041 g of Se (0.5 m mol) to 3 ml TOP in a bottle. Heat the solution to about 150 degrees C to dissolve the Se.

 

CdSe11

The CdO, TDPA and TOPO mixture initially becomes a red solution. Continue heating it.

 

CdSe12

It takes anywhere between 1 - 2 hours for the solution to become clear. Once the solution is clear, drop the temperature to 260 degrees C.

 

CdSe13

Using a syringe, take about 2 ml of TOP-Se solution.

 

CdSe14

Inject the solution into the round bottom flask and start observing the color change.

 

CdSe15

The solution turns yellow and start taking aliquots (remove a small amount of solution and place in seperate vials after mixing) every 30 sec - 1 min depending on the rate of reaction. Obtain 6-7 samples. You can take aliquots at 1 min, 2 min, 5 min, 10 min.

 

CdSe16

The solution turns orange in a minute or so.

 

CdSe17

The solution color gets darker with time.

 

CdSe18

The solution becomes deep red in color in about 10-15 minutes after the TOP-Se solution is injected.

 

CdSe19

You can shine a broadband fluorescent source behind the vial and take a picture.

 

CdSe20

Dilute the CdSe nanoparticle solutions (approximately 1:3 with hexane). Line them in a row and take a digital picture. Also record the absorbance spectrum of the solutions to find the peak wavelength. Include these in your write-up.


QUESTIONS:

  1. How does the absorption spectrum of the aliquots change from samples that were taken from the solution at earlier times (for example, 10 s) compared to later times (60 s)?
  2. Why do the samples exhibit different colors since they are all the same material?

Synthesis of Au Colloids

Synthesis of Au colloids

This experiment demonstrates the size-dependent optical properties of gold nanoparticles. Nano-sized gold particles will be synthesized and then used as a salt sensor. This procedure is adapted from "Color My Nanoworld." (McFarland, Adam D.; Haynes, Christy L.; Mirkin, Chad A.; Van Duyne, Richard P.; Godwin, Hilary A. J. Chem. Ed. 2004, 81).

BACKGROUND:

All physical and chemical properties are size-dependent over a certain size regime specific to the material and property of interest. When materials, like silver or gold metal, are similar in size to the wavelengths of visible light (400-750 nm), they interact with light in interesting ways. Accordingly, the apparent color of a solution of silver or gold nanoparticles depends strongly on the size and shape of the constituent nanoparticles. The volume (and shape) of a nanoparticle determines how it interacts with light, and the color observed from the nanoparticles. For example, while bulk gold is yellow, a solution of nano-sized particles of gold can appear to be many different colors, depending on the size and shapes of the nanoparticles.

MATERIALS:

  • HAuCl4
  • Sodium Citrate (Na3C6H5O7)
  • NaCl
  • Beaker, measuring flask, spatula, glass vials, stir/hot plate, stir bar

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
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step-2AuColloid

Prepare 1 mM HAuCl4 solution by dissolving of HAuCl4 in 500mL of distilled water and pour into a beaker.

 

step3-AuColloid

Prepare a 38.8 mM sodium citrate by dissolving the solid in 50 ml of distilled water.

 

step4-AuColloid

Pour 20 ml of 1 mM HAuCl4 into a 50 ml beaker.

 

step5-AuColloid

Add a magnetic stir bar and heat the solution to boiling on a stir/hot plate while stirring.

 

step6-AuColloid

After the solution begins to boil, add 2 ml of 38.8 mM Na3C6H5O7.

 

step7-AuColloid

Continue to boil and stir the solution until it is deep red color (about 10 min). As the solution boils, add distilled water to keep the total solution volume near 22 ml.

 

step8-AuColloid

When the solution is a deep red color, turn off the hot plate and stirrer and let the solution cool to room temperature.

 

step10-AuColloid

In each of the three glass vials, place 3 mL of the gold nanoparticle solution and add 3 mL of distilled water to each vial.

 

With a dropper, add 5-10 drops, one at a time of the salt solution to one of the vials.

 

step12-AuColloid

With a dropper, add 5-10 drops, one at a time of the sugar solution to one of the vials.

 

 


QUESTIONS:

  1. Based on the fact that the citrate anions covers the surface of each nanoparticle, explain what keeps the nanoparticles from sticking together (aggregating) in the original solution.
  2. Why does adding the salt solution produce a different result from adding the sugar solution?
  3. How could the effect in part B be used to detect the binding of biomolecules, such as DNA or antibodies, that stick to one another or to other molecules? How could these molecules be used to cause aggregation of the nanoparticles?

NANOSCALE DEVICES

Organic Light Emitting Diodes

Organic Light Emitting Diodes (OLED)

The main objective of this experiment is: (1) to synthesize molecules that exhibit electroluminescence and (2) to measure their properties in a sandwich device structure

BACKGROUND:

The basic organic light emitting diode (OLED) cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers consist of (i) a hole-injection layer, (ii) a hole-transport layer, (iii) an emissive layer, and (iv) an electron-transport layer. When a voltage is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device. OLEDs can be very thin (the active area producing the light is several hundred nm) and they have a wide viewing angle (up to 160 degrees). They have currently found use in portable devices such as cellular phones, digital video cameras, DVD players, and PDAs. OLED display technology can also be found in car audio components and cellular phones.

In this experiment, we are only looking at an organic molecule layer that can exhibit electroluminescence (there are no hole or electron transport layers). This layer is the emissive layer. Under an applied voltage, electrons are injected through the cathode (GaIn alloy) and holes are injected through the anode (indium tin oxide - ITO); charge recombination in the Ru complex results in light emission.

MATERIALS:

Synthesis:

  • » Tris-(2,2' bipyridyl)dichlororuthenium(II) hexahydrate
  • » Sodium tetrafluoroborate (Aldrich 20,221-5)
  • » Ethanol

OLEDs:

  • » Indium Tin Oxide (ITO) - coated slides
  • » Acetonitrile
  • » Cotton swab
  • » Liquid gallium indium alloy
  • » Sodium hydroxide
  • » Power supply
  • » Hersch Funnel
  • » Vacuum Filtration flask

Synthesis of Elecroluminescent Molecules

  1. Prepare 45mM Tris-(2,2'-bipyridal)dichlororuthenium(II) hexahydrate. Dissolve 0.6030 g Tris-(2,2'-bipyridyl) dichlororuthenium(II) hexahydrate into 18 mL ddH2O in 30 mL beaker. Heat at 100°C.
  2. Prepare 2M NaBF4. Dissolve 0.6686 g NaBF4 in 3 mL ddH2O. Heat to 100°C.
  3. Prepare RuBPY solution. Add the NaBF4 solution dropwise over 1 to 2 minutes to the Ru(II) solution. Continue stirring for 5 min at 100°C. Turn off the hotplate and allow solution to slowly cool. RuBPY crystals will precipitate. Cool on an ice bath for 20 min and then vacuum filter. Wash three times with 1-mL portions of ice cold ethanol.
  4. Dry crystals in oven at 105°C overnight.

PROCEDURE: (Click on pictures to view the videos)

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OLED1

Identify the conducting side of the ITO glass by using a multimeter to measure resistance. The conducting side will have a resistance in the order of 20-50 Ohms. In this case, the display reads a voltage of 4.00 V and 0.08 Amp when in contact.

 

OLED2

If you are on the wrong side, it should read no current as seen in the display.

 

OLED3

Clean the ITO glass with ethanol and then dry with nitrogen.

 

OLED4

Bake it on a hot plate for 10 min. at 110 oC.

 

OLED5

Dissolve 10 mg [Ru(bpy)3](BF4)2 in 1 mL of acetonitrile. Drop a few drops of the RuBPY solution onto the conducting side of the ITO-coated glass slides. In a fume hood [or dessicator], evaporate the solvent. To get a more uniform coating, try controlling the airflow nitrogen. Place the slide on the hotplate and bake for 5 min.

 

OLED6

Use a syringe to deposit a small bead of liquid gallium-indium alloy on both the Ru-complex and a portion of the ITO-coated slide without the molecules. (This eutectic mixture of 75.5% gallium and 24.5% indium is a liquid above 16.5°C.)

 

OLED7

Touch the positive lead of a 6-volt power supply to the tin-oxide glass (not the [Ru(bpy)3](BF4)2 coating). Gently touch the negative lead to the gallium-indium alloy.

 


QUESTIONS:

  1. Identify the conducting side of the ITO glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 Ohms.
  2. What other types of molecules can be used for in OLEDs? Search the literature and list three others.
  3. Why is a 6 V power supply needed? What would happen if the voltage were less than this value?

Photonic Colloidal Crystals

Photonic Colloidal Crystals

In this lab, photonic crystals are assembled via convective self assembly of polystyrene microspheres. They spontaneously form under ambient conditions of temperature and pressure. Convective assembly uses capillary forces at the meniscus of a colloidal solution and substrate to draw colloids into ordered arrays. These are then compared with the commercially available synthetic opals. Gemstone opal diffracts visible and near IR as a result of sub micrometer size of the colloids.

MATERIALS:

  • » Polystyrene sphere sizes: 0.3560 μ, 0.465 μ, 0.535 μ (Polysciences Inc) and are about 2.6 wt % in solution.
  • » Glass Slides, Micropipette, Petri dish (to slow evaporation and tilt samples)

PROCEDURE: (Click on pictures to view the videos)

*You will need QuickTime Player installed on your computer to view the videos.
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The procedure is as simple as evaporation of a solution on a substrate, as outlined.


 Cleaning the glass slides
PC1

You will be provided with glass slides that have been treated with piranha etch (1:5 ratio of sulphuric acid to hydrogen peroxide) for 30 min to obtain a clean and hydrophilic surface.

To begin with, the microscope glass slides were placed in a beaker to which 50 ml of sulphuric acid was added in a fumehood.

PC2

10 mL of hydrogen peroxide was added to the beaker containing the glass slides and sulphuric acid.

 

PC3

The reaction is exothermic with gas release.

Warning: Piranha solution is very energetic and potentially explosive.

 

PC4

After 30 minutes, the piranha solution was emptied into a waste container while retaining the glass slides in the beaker. The piranha solution container remained open in the hood until cool. NEVER ADD PIRANHA TO ORGANIC WASTE!!!!

 


Assembling Spheres on a Surface
PC5

Wash the glass slides with water multiple times and then dry with nitrogen. Glass slides can be stored in ethanol until used.

 

PC6

Polystyrene beads with various diameters (between 0.3 and 0.6 μm) with no surface treatment and monodispersed in pure water are obtained from Polysciences Inc . They are filtered using Wattman filters to ensure uniform size distribution.

 

PC7

Place the clean glass slide in a petri dish and add about 6-8 ml of the polystyrene mixture with a proper ratio of water / particles (depending on the deposition area and the size of the spheres) using a syringe with the filter attached on the glass slide.

 

PC8

Tilt the solution around carefully so that it covers the slide without leaving any gap.

 

PC9

Cover the petri dish (small volume helps in slowing down the evaporation and also the effect of external airflow). The whole system is tilted at about 9o to ensure that the evaporation starts from the top of the sample and proceeds to the bottom. It takes 2 hours or leave it overnight to dry completely.

 

PC10

Within a minute or so, you can observe that excess solution accumulates at the bottom of the slide.

 

PC11

This can be drained with a syringe without disturbing the glass slide.

 

PC12

Repeat the procedure for the different polystyrene diameters and compare the difference in color with the sizes used.

 


 Imaging the Photonic Crystal
Look at the sample using SEM. The sample needs to be sputter-coated (platinum/palladium alloy) to provide a conducting path for the electrons and to avoid sample charging effects before mounting in the SEM.

QUESTIONS:

  1. Compare the SEM images of polystyrene microspheres of different diameters. What did the ordering look like? Were you able to see multi-layer ordering?
  2. What colors were observed from the different sizes of spheres assembled into a crystal?
  3. What did the ordering look like for the solution with no dilution and a 1:3 dilution?

Last Updated on Friday, 10 December 2010 15:19
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