3D Micro/Nano-Sculptures made of a Single Walled Carbon Nanotube Polymer Matrix via Two Photon Polymerization

Catalyst Volume 6

Abstract

The objective of this study was to fabricate 3D micro/nano-structures with an even distribution of single-walled carbon nanotubes (SWCNTs). We utilized two-photon polymerization by means of a pulsed femtosecond laser for fabrication and Raman microscopy for the detection of carbon nanotubes in the structures. In order to test the capability of our laser, we fabricated an assortment of interesting microstructures such as an 8 micron gear, 7 micron dog, and a 100 nm diameter nano-wire. We believe that this study will open the door to the fabrication of even more intricate micro-structures with an even embedment of carbon nanotubes for practical purposes.

Introduction

Two-photon polymerization (TPP) is a well-established method used to fabricate intricate 3D micro/nano structures from polymers. These 3D structures have vast potential in applications such as micro-electromechanical devices, sensors, and targeted drug delivery systems. However, it remains necessary to functionalize and enhance the mechanical properties of these polymer micro-structures for practical applications. To this end, single-walled carbon nanotubes (SWCNTs) are critically acclaimed as ideal fillers to enhance mechanical properties of the polymer due to their high Young’s modulus (up to 1.5 TPa), high tensile strengths (up to 63 GPa), high aspect ratios (up to 10⁶), and small diameters (~ 1 nm) ¹. In this report, we establish a novel way to evenly embed SWCNTs into 3D micro/nano polymer structures by means of TPP.

Dispersing SWCNTs Evenly in Photoresin

Our photoresin consisted of 0.01 weight % SWCNTs, 96.67 wt % of R712, and 1.67 wt % of photoinitiator and photosensitizer. SWCNTs were evenly distributed in the photoresin by sonicating the mixture for 2-3 hours. After taking the UV spectrum of the photoresin (as seen in Figure 2), we observed a high absorption below 390 nm and low absorption in the IR region, enabling the photoresin photopolymerizable for two-photon polymerization.

Two Photon Polymerization

A Ti:Sapphire pulsed femto-second laser at 780 nm and an intensity of 7 mW was utilized to excite two-photon absorption on the SWCNT dispersed UV photo-polymerizing resin. It took two photons to successfully photo-polymerize the resin, because our photo-resin exhibited a high absorption at 390 nm, and our laser operated at 780 nm. This only occurred at the focus spot of the laser where the photon intensity was highest (see Figure 3). In this manner, point-by-point nanometric volumes of photo-resin were photopolymerized following the trajectory of the focus spot (whose movement was dictated by a pre-programmed CAD file). An assortment of interesting micro/nano-sculptures with an even dispersion of SWCNTs was obtained.

Fabricated Microstructures

Figure 4 shows a few examples of micro-sculptures that I fabricated this summer using two-photon polymerization. All of these structures exhibited a uniform dispersion of SWCNTs.

SWCNTs Embedded in Micro/Nano-Structures

The uniform embedment of SWCNTs in our fabricated micro/nano-structures was detected by Raman Microscopy instrumentation (Raman-11).

To this end, a laser at a wavelength of 785 nm, 10 s exposure time, and intensity of 0.56 mW was utilized. Our sample was placed on a glass substrate and exhibited Raman scattering when eradiated with the laser. The Raman scattering was separated from the Rayleigh scattering through a filter and split into its different frequencies at a grating. The CCD camera then analyzed the Raman signal and constructed a graph of Raman signal vs. wavelength. By using our computer, we were able to input a certain frequency and assign a color to the corresponding Raman intensity at that frequency. We selected the Raman intensity at the G-band frequency to be white (1590 cm^-1) and every other Raman intensity signal to be black. The white color then represented a high G-band intensity (hence, a high concentration of SWCNTs). From this information, point-by-point, the computer reconstructed our Raman image by analyzing whether or not the sample had a high Raman intensity signal and assigned a white color to that pixel. If the image resembled our sample, then we had a uniform dispersion of SWCNTs.

As seen in Figure 5, by comparing the SEM image, Brightfield image, and Raman image of the micro bull and nanowire, we observed a uniform dispersion of nanotubes. Furthermore, by taking the Raman spectra on and off the structure, a sharp peak at the G-band occurred on the structure, while no signal occurred off the structure indicating nanotubes only exist on the structure.

Alignment of SWCNTs in Nano-wires

SWCNTs were found to be aligned along the axis of the fabricated nano-wires as observed by polarized Raman microscopy. To this end, we observed that the strongest Raman signal arose when the nano-wire was parallel to laser polarization, and the lowest signal occurred when the nano-wire was perpendicular. Figure 6 shows the G-band Raman intensity vs. degree of nano-wire orientation. While SWCNT alignment may be due to spatial confinement, the reason behind their alignment is still being researched by my research team at Osaka University today.

Conclusion

We have successfully demonstrated 3D micro/nano fabrication of a SWCNT polymer matrix using TPP and elucidated that SWCNTs were embedded in micro structures and aligned along the nanowire axis. We believe that these methods will open the door to a variety of applications needing SWCNT reinforced micro-structures such as drug delivery devices, sensors, and MEMs in the future.

References

1. Coleman, J.N. Carbon 44. 2006, 1624–52.
2. Ichida, M. Appl. Phys. A. 2004, 78, 1117–20.

The Ghostly Haunting of Limb Lost

Catalyst Volume 6

The brain’s neural pathways are like a city’s infrastructure. Once the routes and support structures are firmly in place, it is difficult to remove them to construct a new route. This helps explain amputees’ reports of phantom limbs and the painful sensations they radiate. How much of the pain is real and how much is psychological has yet to be determined, but treatments address both sources.

The phantom limb was first documented by Dr. S. Weir Mitchell after observations with Civil War amputees.¹ It is a fascinating enigma that has appeared in literature: Captain Ahab’s missing leg in Herman Melville’s Moby Dick, Captain Hook’s lost hand in J. M. Barrie’s Peter Pan, and Long John Silver’s absent leg in Robert Louis Stevenson’s Treasure Island. Why does the brain yearn for the absent limb so much that fantasizes emerge? The answer may reside in ascending sensory pathways from the peripheral nervous system. Once established, the brain finds it difficult to change expected input from these neural pathways.

During infancy, the brain examines the body to understand itself spatially and topologically, building upon this image from the senses throughout life. Interestingly, those who undergo amputations in infancy experience neither the sensation nor the pain of phantom limbs because the missing limbs had not been there long enough to establish a solid pathway.² However, for those that retain their limbs, the development of the senses in early childhood is faster than at any other point. Changing body image at an advanced age is too drastic and demanding for the brain. One contributing factor is the elderly’s diminished brain size. On average, the brain loses 5-10% of its weight between the ages of 20 and 90, with a higher proportion lost with increasing age.³ In addition, the grooves on the surface widen while the swellings and depressions become smaller. Deep grooves in the brain indicate increased surface area for synapses, the connecting space between neurons, to form. Moreover, the formation of neurofibriallary tangles, decayed portions of the dendrites receiving the sensory information from other neurons, impede information transmission.³ Finally, abnormally hard clusters of damaged or dying neurons, known as “senile plaques,” emerge and accumulate. Neurons are not replaced when they die, so as one gets older, one literally has less to work with. Thus, with decreased plasticity, the body image becomes fixed with one’s brain regressing to the stage formed in earlier years.

This pathway, however, is not indestructible because amputees report that phantom limb sensations decrease with time. Due to the plasticity of the brain, the brain takes time to “rewire” itself by abolishing old connections in favor of new, useful connections elsewhere. For example, after an amputation, patients often describe the entire appendages, with the most awareness at the distal (end) portions of the limb (i.e. fingers and toes compared with the forearms and calves, respectively).^4,5 This is because distal anatomical structures contain the greatest amount of sensory nerves and command a larger portion of the somatosensory cortex. In time, however, the phantom limb perception shrinks until it disappears into the stump.^6-8

These concepts are visualized by the sensory homunculus (Figure 1), where the size of the appendage reflects the sensitivity and thus concentration of neurons there. Thus, infants use their hands, lips, and tongue frequency in order to shape and understand their world. Since more neurons are dedicated to these extremities, it takes longer to rewire the corresponding pathways. Instead, the brain completely rewires the proximal portions of the limb so that the phantom sensation in the length of the appendage seems to shrink faster than the distal portions.

When subjects encounter identical stimuli, the sensation experienced is usually comparable between them. For example, when we touch a pot on a lit stove, we feel burning and not tickling. With amputees, this precedence doesn’t hold. Each amputee’s phantom limb is unique: it can feel authentic and present but fake, painful, or painless. There is little to suggest that patients are lying about the pain, yet it is well known that the brain frequently tricks the body.

Psychological pain can also manifest itself as physical pain. Amputee patients who feared an inability to recover were hostile to and jealous of other members of society, consequently experiencing pain in the phantom limb with these heightened emotions. However, once these patients underwent therapy and obtained a positive attitude, the pain faded.²

Traditional approaches to alleviating pain, such as injection of nerve blocks, myoelectric prosthesis, and cordotomy, have been more procedural.^9,10 A nerve block is an injection of a local anesthetic to stop transmission of a message along the nerve so that the brain never receives the pain signal from the stump. A myoelectric prosthesis is an artificial limb, which uses electronic sensors to translate muscle and nerve activity into the intended movement. While the brain is manipulated into replacing the phantom limb with an artifical one, prosthetics often do not alleviate phantom pain. One theory for this is that since visual sensory information contradicts tactile sensory information, the brain refuses to be tricked. Cordotomy is the most invasive of the procedures listed because it requires a neurosurgeon to disable certain rising tracts in the spinal cord. Thus, it is only employed in severe cancer- or trauma-related cases. Despite the variety of approaches, the results are slightly effective at best.⁹

Recently, researchers have turned to mind-body therapies to relieve chronic phantom pain, yielding tentatively successful results. A review by Dr. Vera Moura of the Department of Physical Medicine and Rehabilitation on Integrative Medicine at University of North Carolina Hospitals tied together studies that used hypnosis, guided imagery, and biofeedback (such as visual mirror exercises).¹¹ These non-invasive mind-body alternatives consider the psychological aspect of pain. Hypnosis has been found to reduce postsurgical pain, so researchers attempted to transfer its effects to amputees.¹² In several studies, arm amputees varying in sex and age saw a reduction in pain frequency and intensity after attending hypnosis sessions.^13-15 These studies indicate that mind can truly triumph over matter, but caution must be taken because trial sizes were small and hypnosis is a murky field. Therefore, more research is necessary before any definite conclusions can be made.

Guided imagery is another mind-body approach that extends beyond the typical denotation of our senses, and it utilizes more neural pathways than normal to create a memorable, mental image. This treatment combines interactions between patient and therapist and patient and body image.⁹ In Zuckweiler’s experiment, 14 patients with diverse backgrounds had 5 to 15 imagery sessions, during which they attempted to reprogram their minds to accept the new body form.¹⁶ Patients were taught Zuckweiler Image Imprinting (ZIP), which involves taking an object and storing it as a mental image. They were then asked to compare their phantom limb pain to the object in their mental image and switch the sensations associated with the two objects. Over time, as the phantom sensation decreased by using different mental images, the discrepancy between the new body image without the limb and old body image with the limb was reconciled. Zuckweiler’s study showed successful pain intensity reduction within only six months. ZIP forces patients’ minds to accept their new bodies. Since his method encompasses visual, auditory, and kinesthetic learning, customized treatment allows patients to comprehend and create new connections.

The final mind-body approach is biofeedback, of which there are two popular kinds. Thermal biofeedback teaches patients to increase the peripheral skin temperature at the stump.¹⁷ This seems unlikely, since body temperature is an autonomic function along with vital processes such as heart rate and breathing. In some instances, however, an individual can have partial, conscious control. Although the hypothalamus is responsible for standard body temperature of 37.0°C (98.6°F), it is possible for consciousness to affect peripheral skin temperature. Successful patients begin to link skin temperature with pain.¹⁸ Physiologically, the regulation of one function often results in the coupling of the response to a stimulus. For example, thermal biofeedback was coupled with breathing relaxation techniques, which caused the temperature of the stump to increase and relax, decreasing the pain and thus increasing the patient’s ability to contend with remaining pain. It is unknown, however, if thermal biofeedback is an effective treatment for all phantom pain; like most areas of science and medicine, more research is needed.

The second biofeedback type, visual mirror feedback (VMF), uses a box with mirrors to fool the brain. A rectangular box with no top and two holes for each arm (or leg) is set in front of a patient. In the middle of the box is a one-sided mirror septum facing the limb that is intact (Figure 2). Patients are thus presented with the illusion that both appendages are whole. Dr. Ramachandran, the inventor of this technique, conducted a study in which 10 amputee patients were treated with VMF in six sessions of 5 to 15 minutes a day for several weeks.¹⁹ Every patient had a positive reaction that included reduced pain, pain intensity, mobility restriction, and spasms. Once again, there was a conscious effort to train the brain, so patients were able to redirect unpleasant sensations. This therapy is almost opposite to ZIP since the patient is picturing the limb as whole to alleviate the pain rather than ignoring it. VMF treatment is one of the most common due to its success amongst many different amputees.

A theory behind mind-body approaches’ emerging successes is the conscious effort patients put forth to overcome pain. In previously mentioned traditional procedural methods, patients passively receive a certain treatment and hope to obtain a positive result. In some cases, there are even negative side effects; for example, a nerve block may lead to rashes, itching, and an abnormal rise in blood sugar. Invasive procedural approaches like the cordotomy can only be attempted once. Mind-body approaches can be practiced, optimized over time, and are much safer than procedural methods.

Understanding of phantom pain has progressed significantly since its initial documentation during the Civil War. Traditional procedural methods to treat it have been developed, but recently, the psychological aspect of pain and sensation has been addressed in mind-body methods. Unfortunately, neither approach has achieved complete success, partially because of the individualistic nature of phantom limbs and the associated pain. The neurological explanations behind both phenomena are relatively unknown, but it is agreed the ghostly perceptions are a mixture of psychological and real sensations. Perhaps the most effective treatments are those that address both.

References

1. Lehrer, J. Proust Was a Neuroscientist; Houghton Mifflin: Boston, 2008
2. Kolb, L. C. The Painful Phantom, Psychology, Physiology, and Treatment; Charles C. Thomas: Springfield, Illinois, USA, 1954.
3. Guttman, M. The Aging Brain. USC Health Magazine http://www.usc.edu/hsc/info/pr/hmm/01spring/brain.html (Accessed Jan. 22, 2013).
4. Newton, A. Somatosensory Map. http://www.alinenewton.com/neuroscience.htm (Accessed Jan. 18, 2013).
5. Pain and Touch, Handbook of Perception and Cognition. 2nd ed. Lawrence Kruger, Ed.; Academic: San Diego. CA, 1996.
6. Jensen, T. S. Pain. 1985, 21, 267-78.
7. Hunter, J. P. Neuroscience. 2008, 156, 939-49.
8. Desmond, D. M. Int. J. Rehabil. Res. 2010, 33, 279-82
9. Lotze, M. Nat. Neurosci. 1992, 2, 501-2.
10. Pool, J. L. Ann. Surg. 1946, 124, 386-91.
11. Moura, V. L. Am. J. Phys. Med. Rehabil. 2012, 8, 701-14.
12. Black, L. M. J. Fam. Pract. 2009, 58, 155-8.
13. Oakley, D. A. Clin. Rehabil. 2002, 18, 84-92.
14. Bamford, C. Contemp. Hypn. 2006, 23, 115-26.
15. Rickard, J. A. Ph.D. Dissertation, Washington State University, Pullman, WA, 2004.
16. Zuckweiler, R. JPO. 2005, 17, 113-8.
17. Sherman, R. A. Am. J. Phys. Med. 1986, 65, 281-97.
18. Shaffer, F.; Moss, D. Textbook of Complementary and Alternative Medicine; 2nd Ed. Informa Healthcare: London, UK, 2006.
19. Ramachandran, V. S. Brain. 2009, 132, 1693-710.
20. Trivialperusal. Sensory Homunculus. http://trivialperusal.files.wordpress.com/2011/04/sensory_homunculus.jpg (Accessed Jan. 18,         2013).
21. Phelan, L. Mirror Box Therapy. http://farm3.static.flickr.com/2567/3927573088_aa057fcc61.jpg (Accessed Jan. 18, 2013).

The Bridge from Discovery to Care: Translational Biomedical Research

Catalyst Volume 6

Since the 1970s, both the number of molecular biology PhD scientists and the amount of biomedical research have grown rapidly, greatly expanding our knowledge of the cell.¹ This explosion has led to incredible scientific achievements, including development of the polymerase chain reaction in the 1980s and completion of the Human Genome Project in 2003.^2-4 The focus of research has shifted from single genes to all genes, from single proteins to all proteins. Neither scientists nor pharmaceutical companies, however, have been able to keep pace with the sheer quantity and complexity of modern biomedical research. Additionally, while the majority of medical researchers were once physician-scientists in the 1950s and 1960s, they are predominantly PhDs today.¹ Questions of basic and clinical research, once addressed side by side, are now separate.

The widening gap between scientific discovery and therapeutic impact is a result of these changes. In the United States, the dramatic increase in spending for pharmaceutical research and development has been offset by a disappointing decrease in therapeutic output (Figure 1). As this paradox becomes more apparent, translational research, which aims to convert laboratory findings into clinical successes, emerges as an increasingly important endeavor.^5,6

In 2006, the U.S. National Institutes of Health (NIH), the largest source of funding for medical research in the world, focused its attention on translational research by launching the Clinical and Translational Science Awards program.^7,8 However, implementing effective translational research is both time- and labor-intensive. According to Dr. Garret FitzGerald, Director of the Institute for Translational Medicine and Therapeutics at the University of Pennsylvania, challenges include a lack of human capital with translational skill sets, relevant information systems, and intellectual property incentives.⁹

During his leadership of the NIH from 2002 to 2008, Dr. Elias Zerhouni witnessed the consequences of clinicians lacking in training on the speed of scientific advancements for patient care.^10,11 Beyond the need for manpower, an open culture of communication between scientists and clinicians is necessary.

Drug development is a one-way process from benchside to bedside in which scientists identify drug targets, conduct clinical tests, and develop marketable drugs. Many argue, however, that the communication must run in the opposite direction, too; feedback from clinical trials and doctors is valuable because understanding their concerns allows researchers to improve drug development.¹² The third challenge derives from current institutional practices and regulations. An investigator’s publication record rather than their efforts to advance medicine determines success.¹³ Research funding is also granted on an individual basis, which does not promote the collaboration necessary for successful translational research. Lastly, the regulatory and patent processes governing drug development require much expertise and time to navigate, which offer little incentive for researchers to become involved.¹

To better integrate basic science with clinical science progress, countries such as the United States are building a new team of leaders in all aspects of clinical research: medicine, pharmacology, toxicology, intellectual property, manufacturing, and clinical trial design and regulation.¹³ Dr. Francis Collins, Director of the NIH since 2009, has called for a partnership between academia, government, private, and patient organizations to repurpose molecular compounds previously failing in their original use.^15,16 Historically, Collins referred excitedly to azidothymidine, a drug originally developed to treat cancer that later treated HIV/AIDS.¹⁴ Tremendous potential lies in applying scientific developments to other contexts, and the NIH has already drafted policy for this purpose.¹⁵

However, the growing support for translational research does not diminish the importance of basic scientific research, which poses the most interesting questions. Translational biomedical research creates an efficient environment for scientists to work at the interface of basic science and therapeutic development and to help fulfill the social contract between scientists and citizens. The full impact of translational initiatives has yet to be seen because the success of drug development, which can take up to 20 years, cannot be evaluated easily or quickly. For now, we can hope that integrating the work of scientists and clinicians will benefit both the patients, who await treatment, and the researchers, who only dream of seeing their discoveries transformed into new therapies for disease.

References

1. Butler, D. Nature. 2008, 453, 840–2.
2. Smithsonian Institution Archives. Smithsonian Videohistory Collection: The History of PCR (RU 9577). http://siarchives.si.edu/research/videohistory_catalog9577.html (Accessed Jan. 15, 2013).
3. National Center for Biotechnology Information (NCBI). Probe, Reagents for Functional Genomics: PCR. http://www.ncbi.nlm.nih.gov/projects/genome/probe/doc/TechPCR.shtml (Accessed Jan. 15, 2013).
4. Human Genome Project Information. About the Human Genome Project. http://www.ornl.gov/sci/techresources/Human_Genome/project/about.shtml (Accessed Jan. 15, 2013).
5. CTSI (Clinical and Translational Science Institute) at UCSF. Translational Medicine at UCSF: An Interview with Clay Johnston. http://ctsi.ucsf.edu/news/about-ctsi/translational-medicine-ucsf-interview-clay-johnston (Accessed Jan.15, 2013)
6. Helwick, C. Anticancer Drug Development Trends: Translational Medicine. American Health & Drug Benefits. http://www.ahdbonline.com/article/anticancer-drug-development-trends-translational-medicine (Accessed Jan. 15, 2013).
7. National Institutes of Health (NIH). About NIH. http://www.nih.gov/about/ (Accessed Jan. 15, 2013).
8. National Institutes of Health National Center for Advancing Translational Sciences (CTSA). About the CTSA Program. http://www.ncats.nih.gov/research/cts/ctsa/about/about.html (Accessed Jan. 15, 2013).
9. Pers. comm. Dr. Garret FitzGerald, Director of the Institute for Translational Medicine & Therapeutics at the University of Pennsylvania.
10. NIH News. Elias A. Zerhouni to End Tenure as Director of the National Institutes of Health. http://www.nih.gov/news/health/sep2008/od-24.htm (Accessed Jan. 15, 2013).
11. Wang, S.S. Sanofi’s Zerhouni on Translational Research: No Simple Solution. The Wall Street Journal. Health Blog 2011 http://blogs.wsj.com/health/2011/05/20/sanofis-zerhouni-on-translational-research-no-simple-solution/ (Accessed Jan. 15, 2013).
12. Ledford, H. Nature. 2008. 453, 843-5.
13. Nature. 2008, 543, 823.
14. TEDMED 2012. Francis Collins. http://youtu.be/spUoPC_TU_8 (Accessed Jan. 15, 2013).
15. Wang, S. Bridge the Gap Between Basic Research and Patient Care, NIH Head Urges. The Wall Street Journal Health Blog. http://blogs.wsj.com/health/2012/04/11/bridge-the-gap-between-basic      research-and-patient-care-nih-head-urges/ (Accessed Jan. 15, 2013).

The Illusion of Race

Catalyst Volume 6

Race is one of the most pervasive features of American social life; neglecting the concept of race would be like questioning the existence of gravity. Though we would like to consider our nation a post-racial society, we still place great importance on race by asking for it on forms ranging from voter registration to the PSAT. However, many would be surprised to realize that race does not have a biological basis – there is no single defining characteristic or gene that can be unequivocally used to distinguish one race from another.¹ Rather, it is a manmade concept used to describe differences in physical appearance. Yet, we have internalized the social construct of race to such a degree that it seems to have genetic significance, masking the fact that race is actually something we are raised with. That a simple internalized ideology creates disparities in contemporary American society, from socioeconomic status to healthcare accessibility, illustrates the urgency of exposing this myth of race.

Throughout American history, racial connotations have been fluid, with different ethnographic groups falling in and out of favor based upon societal views at a given time. Race was originally conceived as a way to justify colonialism. European colonizers institutionalized their ethnocentric attitudes by creating the concept of race in order to differentiate between the civilized and the savage, the Christians and the heathens. This dichotomy facilitated mercantilism, the economic policy designed to accrue astronomical profits for the European countries through the exploitation of “inferior” races. Scholars of Critical Race Theory show, more generally, that the boundaries of racial categories shift to accommodate political realities and conventional wisdom of a given time and place.²

This definition of race changed in the United States over the centuries. For example, when the Irish and Italians first immigrated in the early 20th century, they were seen as “swarthy drunkards” – clearly not part of the white “elite.” Within two generations, however, these same people were able to assimilate into the Caucasian-dominated culture while African-Americans were still considered a separate entity. Similarly, during the era of the Jim Crow laws, courts had the power of determining who was black and who was not; in Virginia, a person was considered to be black if he or she was at least 1/16th African-American; in Florida, a black person was at least 1/8th African-American; and in Alabama, having even the smallest sliver of African-American heritage made a person black.³ Thus, a person could literally change race by simply moving from one state to another. Today, the commonly defined race classifications, as specified by the US Census, include White, Black, Asian, American Indian or Alaska Native, Pacific Islander or Native Hawaiian, Other, and Multiracial. Because there is no scientific cut-offs to determine what race a person is, racial data is largely based on self-identification, which points to its lack of biological legitimacy. For example, 30% of Americans who label themselves as White do not have at least90% European ancestry.⁴

We may think our conceptualization of race is based upon biological makeup, but it is actually an expression of actions, attitudes, and social patterns. When examining the science behind race, most scholars across various disciplines, including evolutionary biology, sociology, and anthropology, have come to the consensus that distinctions made by race are not genetically discrete, cannot be reliably measured, and are not meaningful in the scientific sense.⁵

Some argue that race is a genetic concept based upon a higher incidence of particular diseases affecting certain races. However, purely hereditary diseases are extremely rare. For example, 1/2300 births for cystic fibrosis, 1/10000 births for Huntington’s disease, and 1/3000 births for Duchenne’s muscular dystrophy.⁶ Rather, diseases often reflect shared lifestyles and environments instead of shared genes, because factors such as poverty and malnutrition are also often “inherited” through family lines. Even genetic polymorphisms in hemoglobin, which lead to populations with lower susceptibility to malaria, can be partly explained by environmental factors.^6-8 Thus, diseases traditionally tied to certain races cannot be explicitly attributed to genes, discrediting the idea that races are genetically disparate. Genetic differences are better described as percentages of people with a particular gene polymorphism, which change according to the environment.⁶

Racial groupings actually reflect little of the genetic variations existing in humans. Studies have shown that about 90% of variations in human genetics is present within a population on a continent, while around 10% of genetic variation occurs between continental populations.¹ Variation in physical characteristics, the traditional basis for determining race, does not imply underlying genetic differences. When we internalize the false ideology that race is genetic, we are mistakenly implying that there are human subspecies.

Although race is a social construct, it has a widespread influence on society, especially in the United States. In particular, minorities face disadvantages in numerous areas ranging from healthcare to education.^7,8 Reports about Mitt Romney’s rumored adoption of a darker skin tone when addressing Latino voters or statistics indicating that the median household wealth of whites is 20 times that of blacks reinforces the existence of a racialized society.⁵ This is shocking and disturbing; race may not be real, but its effects contribute to real inequality. Once everyone understands this racial illusion, we can begin making effective change.

References

1. Bamshad, M. J.; Olson, S. E. Does Race Exist? Scientific American, New York City, Nov. 10, 2003, p. 78-85.
2. Calavita, K. Invitation to Law and Society: An Introduction to the Study of Real Law. University of Chicago Press: Boston, MA, 2007.
3. Rothenberg, P. S. Race, Class, and Gender in the United States, 7th ed.; Worth Publishers: New York, NY, 2007.
4. Lorber, J.; Hess, B. B.; Ferree, M. M.; Eds. Revisioning Gender; AltaMira Press: Walnut Creek, CA, 2000.
5. Costantini, C. ABC News. http://abcnews.go.com/ABC_Univision/News/mitt-romneys-tan-draws-media-fire-makeup-artist/story?id=17290303       (Accessed Oct. 26, 2012).
6. Pearce, N. BMJ. 2004, 328, 1070-2.
7. Stone, J. Theor. Med. Bioethics. 2002, 23, 499-518.
8. Witzig, R. The Medicalization of Race: Scientific Legitimization of a Flawed Social Construct. Ann. Intern. Med. 1996, 125, 675-9.
9. Tavernise, S. The New York Times. http://www.nytimes.com/2011/07/26/us/26hispanics.html?_r=0&pagewan (Accessed Oct. 26, 2012).

Terahertz Radiation

Catalyst Volume 6

Abstract

Graphene has received significant attention due to its many unique properties, such as its two-dimensionality, zero-mass and zero-gap band structure, and unsurpassed strength. Additionally, its terahertz (THz) properties are being studied for future ultrafast electronics for information and communication, sensing, and other applications. Although many theoretical studies exist, the basic properties of graphene in the presence of THz radiation are largely unexplored. In this study, we explore the THz dynamics of graphene on an indium phosphide (InP) substrate using THz time-domain spectroscopy (THz-TDS) and laser THz emission microscopy (LTEM). Using LTEM, we compared the THz radiation from InP to the radiation through graphene on InP to find that graphene decreases the amplitude of THz. Furthermore, we investigated the effect of continuous wave (cw) lasers of different wavelengths on THz radiation and discovered that a 365 nm cw laser greatly decreased THz transmittance through graphene on InP. In contrast, an 800 nm cw laser had no effect on transmittance, proving wavelength dependence of THz generation. We also studied the spatial variation of THz absorption of graphene on InP using LTEM, which allowed us to visualize the localized transmittance distribution of graphene. Both THz-TDS and LTEM results help us understand THz functionality in graphene on InP, and this understanding can potentially contribute to the development of future ultrafast electronics.

Introduction

Although microwave and visible light applications are prevalent in electronic and photonic devices, the terahertz (THz) regime (300 GHz to 30 THz) is largely unexplored and unutilized. This “terahertz gap” has captured much attention due to potential applications, such as sensing, communications, and imaging. Graphene has attracted much attention for its unique properties and many potential applications, such as ultrafast electronics, transistors, inert coatings, and biodevices. We are particularly interested in developing ultrafast electronics by utilizing graphene’s absorbance of THz radiation. There are many possible substrates for graphene, and this is the first study to analyze the THz dynamics of graphene on an indium phosphide (InP) substrate. Additionally, we do not know the effect of several phenomena, including the interface effects of graphene on THz absorption and the effect of continuous wave (cw) laser on THz emission and transmission. We aim to characterize graphene on InP, study the interface effects of graphene, and explore the effect of cw lasers on THz emission and transmission. Comparing the effects of different wavelength cw lasers on the THz emission can lead to interpretations that are in agreement with current theories of THz generation. Studying and understanding graphene on different substrates will contribute to the development of ultrafast electronic devices.

Background

Graphene and Indium Phosphide

Graphene is capturing the attention of researchers due to its astounding properties such as its zero bandgap, strength, high mobility, and low resistivity. There are several methods of fabricating graphene, and in this study, we use chemical vapor deposition (CVD) grown graphene.

Because graphene is a single layer of carbon atoms, we must find suitable substrates in order to handle graphene. We are interested in studying graphene on InP because it has a high electron mobility (5400 cm²/(Vs) at 300 K) and a direct bandgap, which may be useful for ultrafast electronics.

Background on Terahertz Radiation

Terahertz radiation is defined as electromagnetic radiation with a frequency in the range of 300 GHz to 30 THz and a wavelength of 1 millimeter to 10 micrometers. In this study, we use two techniques: THz time-domain spectroscopy (THz-TDS) and Laser THz Emission Microscopy (LTEM).

THz-TDS is a technique that probes the properties of a material with pulses of THz radiation. A Ti:Sapphire laser generates an output beam containing a train of femptosecond pulses. The beam is split into two: the pump and probe beams. The pump beam reaches the THz emitter, where the optical pulses are converted into THz electromagnetic pulses. The THz beam goes through the sample and then meets up with the probe beam at the detector, which measures the amplitude and phase of the THz electric field. Fourier analysis of the transmitted THz radiation, as compared with a reference spectrum without a sample, provides the transmission spectrum of the sample.

LTEM measures the near field absorption of the electric field. A pump laser is raster scanned across the sample surface, which produces THz waves. By monitoring the THz amplitude, THz emission or transmission images can be observed.

Methods

In this study, two different systems were used. Both systems can perform THz-TDS and LTEM. Both systems use a beam splitter to split the laser into pump and probe beams. Both use time delay stages, GaAs bowtie-shaped antenna detectors, and lock-in amplifiers. However, the differences are highlighted in Figures 1 and 2.

RESULTS

Data from system 1 led to three major results, but data from system 2 was inconclusive. The first finding from system 1 is that the THz emission decreases more than expected in the presence of graphene on InP. We would expect graphene to cause a 2.3% decrease in THz emission due to its theoretical interband absorbance of 2.3%. However, other factors, such as air, humidity, doping, and inhomogeneity of sample may have caused this decrease to be roughly 28%. Considering that the THz generation mechanism is the surge current effect (surface field effect), it is also possible that this decrease in THz emission is caused by a decrease in band bending when graphene is on the surface of InP, as explained in Figure 5.

The next result is that we can successfully image graphene and see inhomogeneity on the surface of graphene, while the substrate is uniform (Figure 6).

The final result is that a 365 nm cw laser decreases the THz emission, but an 800 nm cw laser has no effect on THz emission. Figure 6 shows this with THz images and Figure 7 shows this with THz-TDS responses. The reason for this wavelength dependence of THz emission is that the THz generation occurs at the surface of the sample, when carriers are excited. The penetration depth of an 800 nm cw laser is too deep and does not interfere with carriers on the surface, but a 365 nm cw laser has a short penetration depth, interfering with carrier interactions at the surface, decreasing THz emission.

DISCUSSION

This research is significant because it is the first THz study that uses InP as a substrate for graphene. We can conclude that graphene affects the surge current mechanism on InP, decreasing the THz emission. The local distribution of the surface of CVD graphene can be visualized using an LTEM system, which is useful for future THz imaging applications in a variety of fields, such as medicine and security. A 365 nm cw laser clearly affects the THz emission mechanism, while an 800 nm cw laser has no effect on THz emission. These results are in agreement with current models because the cw laser has no effect on THz emission when its wavelength is too long; the laser does not interact with carriers on the surface. This helps us understand the THz emission mechanism of InP.

There are several ways to expand upon and improve this research. Firstly, the results of system 1 would be more meaningful if performed in a vacuum and with all other environmental factors held constant. This would allow us to determine the decrease in THz emission caused by graphene alone, which is significant because it gives us insight on the interaction of graphene at the surface of InP (i.e., the decrease in band bending caused by graphene). We also need to compare the THz emission of graphene on InP to that of a mirror in order to know the effect of the reflection from graphene’s shiny surface.

A potentially useful application could be to generate more THz radiation by applying a gate voltage, effectively via a battery. This would be useful in situations where high-intensity THz generation is desired, and it may also be interesting to study graphene with a bandgap tuned by gate voltage.

FUTURE WORK

The data gathered from system 2 were inconclusive, so further study needs to be done in this area. When using system 1 and looking at the THz emission, the 365 nm cw laser causes a significantly greater decrease when graphene is present compared to InP substrate alone (Figure 8). However, when using system 2 and looking at the THz transmission, the 365 nm cw laser causes a significantly greater decrease when graphene is not present (Figure 9). This leads to confusion about how the cw laser is interacting with the surface of graphene and InP.

Although the 365 nm laser clearly affects the THz emission and transmittance, we cannot determine if this effect is primarily from graphene or InP because emission (system 1) and transmission (system 2) results are contradictory. Experiments with cw lasers on InP only and graphene on InP need to be continued in order to fully understand the cw laser effects and its effect on transmission versus emission.

Work by Yuki Sano at the Tonouchi Laboratory at Osaka University is currently doing related research to better characterize the THz dynamics of graphene on InP.

ACKNOWLEDGEMENTS

This research was conducted at Osaka University as part of the NanoJapan program. This material is based upon work supported by the National Science Foundation’s Partnerships for International Research & Education Program (OISE-0968405). Special thanks to the Tonouchi lab members for helping me with this research! Thank you to Sarah Phillips, Junichiro Kono, Cheryl Matherly, and Keiko Packard for organizing this program.

References

1. Inoue, Ryotaro, Kazuhisa Takayama, and Masayoshi Tonouchi. "Angular Dependence of Terahertz Emission from Semiconductor Surfaces Photoexcited by Femtosecond Optical Pulses." Journal of the Optical Society of America B 26.9 (2009): A14. Print.
2. Serita, Kazunori, S. Mizuno, H. Murakami, I. Kawayama, Y. Takahashi, M. Yoshimura, Y. Mori, J. Darma, and M. Tonouchi. "Scanning Laser Terahertz Near-field Imaging System." Optics Express 163639th ser. 20.12 (2012): 1-7. Print.
3. Suzuki, Masato, Masayoshi Tonouchi, Ken-ichi Fujii, Hideyuki Ohtake, and Tomoya Hirosumi. "Excitation Wavelength Dependence of Terahertz Emission from Semiconductor Surface." Applied Physics Letters 89.9 (2006): 091111. Print.