Graphene transistors give bioelectronics a boost.doc

graphene transistors give bioelectronics a boost feb 20, 2013linking neurones with graphenegraphene-based transistors that respond to changes in chemical solutions could be used to link electronic devices directly to the human nervous system. that is the claim of researchers in germany who have built arrays of devices that respond to changes in the electrolytes surrounding living cells. the team hopes that its research could result in retinal implants that could help some visually impaired people see images.the research centres on the small voltage that a neurone creates across its cell membrane when it fires, with the potential difference arising from sodium ions moving into the cell and potassium ions moving out into the surrounding solution. since the 1970s, biophysicists have been trying to detect this sudden change in the electrolytic properties of the liquid surrounding a cell using a type of field-effect transistor (fet). these devices are called solution-gated fets (sgfets) and much of the initial research was done using silicon. but after graphene was isolated in 2004, some researchers realized that this material a layer of carbon just one atom thick could be used to create better sgfets.clean, sensitive and flexibleaccording to jose garrido of the technische universit?t mnchen, who has led the work, graphene offers several important advantages over silicon. first, the graphene surface remains clean unlike silicon, which quickly forms a performance-degrading oxide layer when exposed to the electrolyte. second, electrons in graphene have an extremely high mobility, which makes the device much more sensitive than silicon sgfets. finally, graphene is extremely flexible, which is good because any device implanted within the brain or similar tissue must be bendable.a sgfet is a different take on a conventional graphene fet, in which the current flowing through its graphene channel can be controlled by changing the voltage applied to a nearby gate electrode. in a sgfet, in contrast, the gate voltage is kept constant and the graphene is exposed to the electrolytic environment of the cell. any shift in the concentration of ions in the solution affects the electronic properties of the grapheme, thereby changing its conductivity and the current flowing in the graphene channel. the firing of a neurone is therefore detected as an electronic signal.brainy plansin their new work, garrido and colleagues have created 8 8 arrays of sgfets with each individual transistor measuring about 10 m across. these arrays were used to detect firing signals from neurone cells that were cultured on an artificial medium. the researchers have also shown that neurone cells are able to survive for long periods of time in close proximity to graphene layers. they now want to show that the sgfets work in living tissue rather than cell cultures and that neuronal tissue is not adversely affected by the presence of the devices.according to garrido, an important application of the graphene sgfets would be creating retinal implants that could improve the sight of visually impaired people. indeed, he believes that an array containing about 1000 elements could provide the brain with enough information for a person to be able to perceive an image. another important application could be as cortical implants to help people control artificial limbs.although creating a 1000-element array of graphene sgfets is a straightforward process, garrido says that integrating the technology within a person will require a great deal more work.the technology is described in a preprint on the arxiv server.scientists delve deeper into carbon nanotubesfeb 19, 2013 1 commentone, two, and three walls of carbonthe outer walls of both double- and triple-walled carbon nanotubes (cnts) protect the innermost tubes from interacting with their environment. that is the key finding of a study by researchers in the us, germany and japan, who have made the first detailed examination of triple-walled cnts using resonant raman spectroscopy. the protection afforded by the outer layer allows the tiny tubes to be studied in more detail than ever before, which could be a boon to those using cnts to create new technologies.a single-walled carbon nanotube (swcnt) resembles a tiny drinking straw with a wall that is just one carbon atom thick. a double-walled carbon nanotube (dwcnt) consists of two concentric swcnts coupled together by weak van der waals interactions. the inner and outer tubes can either be semiconducting or metallic. however, because the outer tube is in direct contact with its environment, it can be difficult to obtain accurate information about its fundamental physical properties.third wall protects the secondto gain a better understanding of the outer tube in a dwcnt, thomas hirschmann and paulo araujo at the massachusetts institute of technology and colleagues studied individual and bundled triple-walled carbon nanotubes (twcnts). a twcnt can be thought of as a dwcnt wrapped around a swcnt. the researchers found that the extra outer tube protects the two inner ones from interacting with their environment, thus allowing them to be studied more accurately. an unrolled twcnt can be thought of as a trilayer graphene ribbon, and has all the outstanding electronic and mechanical properties that this carbon material boasts.the team was led by mits mildred dresselhaus and included scientists from the university of hamburg, the nagaoka university of technology and shinshu university. the researchers used a very fast yet sensitive raman spectrometer, which allowed them to detect and characterize the same individual twcnt with different laser lines under identical experimental conditions. only a few groups in the world are equipped with such an instrument capable of characterizing individual cnts in this way, said hirschmann.wall-to-wall measurementsthe analyses allowed us to study fundamental properties such as intertube mechanical coupling, wall-to-wall (wtw) distance, metallicity and curvature-dependent intertube interactions, he explained. such knowledge will be of fundamental importance for technological applications that exploit these nanostructures.the researchers characterized five individual twcnts in detail and found that the wtw distance between the inner two tubes in all the samples ranges from 0.323 to 0.337 nm. these values are larger than the wtw distance observed in previously studied dwcnts (0.2840.323 nm). the distances are also closer to the interlayer distance in graphene (0.335 nm).we also found that the intertube interactions affect innermost nanotubes differently, according to which metallicity they have, and that the elusive mechanical coupling between the radial breathing mode, or rbm, of concentric nanotubes does not exist, even for relatively short wtw distances of 0.323 nm, added hirschmann. this is an important finding and shows that, although the twcnts are hybrid systems, the tubes themselves are mostly independent of one another.wealth of informationthe rbm is the most important spectroscopic signature of a cnt, the frequency of vibration of which is known to be inversely proportional to the tube diameter, he explained. these so-called first-order raman features provide a wealth of information on the electronic and vibrational structure of these nanomaterials.our analyses also shed more light on the van der waals forces mediating the interactions in concentric ordered cnts, such as dwcnts and twcnts, said araujo. these low-energy interactions are important for technology applications because they affect the electronic and vibrational properties of the tubes.the team is now busy analysing shielding phenomena and intertube interaction effects in multi-walled carbon-nanotube systems. here, intertube interactions not only affect the measured rbms but also other raman features. one of our main goals is to find better conditions in which to grow cnts by controlling interactions between nanotubes walls, said hirschmann. to this end, we are working closely with yoong ahm kim and colleagues at shinshu university, who are experts when it comes to synthesizing these nanomaterials.the research is described in acs nano. viewpoint: revisiting thermodynamic efficiencytoma? prosen, faculty of mathematics and physics, department of physics, university of ljubljana, jadranska 19, si-1000 ljubljana, sloveniabreaking time-reversal symmetry in a thermoelectric device affects its efficiency in unexpected ways.+enlarge imagek. brandner et al. 1figure 1 a simple three-terminal thermoelectric device that converts heat into electrical current. the circle is a simple conductor in thermal and electrical contact with two temperature and charge reservoirs (the hotter reservoir is in red, the cooler reservoir in blue.) the third terminal is a probe (yellow), which, on average, does not exchange any heat or particles with the conductor. brandner et al. show that turning on the magnetic field b, can improve the efficiency of the device.thermodynamic engines convert heat into useful work. testing the optimal efficiency of these machines has been at the forefront of scientific developments ever since 1824, when sadi carnot showed that in a simple engine undergoing a reversible thermodynamic cycle, the ratio between used and wasted heat must be less than the ratio of the absolute temperatures of the cold and the hot reservoir. this carnot limit is simply a statement of the second law of thermodynamics.on a practical level, knowing the ideal efficiency of engines and other devices helps us compare the advantages of using and developing one technology versus another. this is particularly important today, as the world faces energy challenges that could be mitigated by using available resources more efficiently. at the same time, studying the efficiency of thermodynamic engines helps us understand fundamental ideas, such as the relationship between the work an engine performs and the information gained or lost in the process, or the implications of a system being microscopically versus macroscopically reversible in time.in this spirit, kay brandner at the university of stuttgart, germany, and colleagues 1 report in physical review letters their calculated efficiency of a simple thermoelectric device that converts heat to electrical current (fig. 1). they show that when the device operates in an external magnetic fielda condition that breaks time-reversal symmetry for the motion of electronsthe efficiency is significantly lower than previous studies predicted. the lower bound on efficiency occurs, they argue, because in addition to the requirement that entropy be greater than or equal to zero, charge must be conserveda point that was missed in earlier work. their findings improve our understanding of thermoelectric efficiency and may one day influence the design of thermoelectric devices for real-world applications.the carnot engine undergoes two isothermal and two adiabatic changes, and runs sufficiently slowly that the thermodynamic state of the engine at each step along the way is well defined. carnot engines have an efficiency c that depends on the temperatures of the two thermal reservoirs providing heat into and out of the engine (c=1-tcold/thot). as typically framed, the carnot cycle is considered macroscopically reversible, since it is infinitely slow and produces no entropy.in contrast, brandner et al. consider the implications of reversibility and irreversibility on a microscopic scale, taking into account the individual trajectories of all of the electrons in a simple thermoelectric device (fig. 1). unlike the carnot engine, this device operates in an out-of-equilibrium steady state, transforming the current of heat to an electric current at a finite voltage.the efficiency of thermoelectric devices like the one brander et al. consider depends on three material properties: the thermopower s, which is the voltage produced by a temperature difference, the electric conductivity , and the heat conductivity . how the combination of these material properties combine to determine the thermoelectrics efficiency can be found from the so-called onsager matrix lij, which connects the heat current (jq) and electrical current (je), to a pair of thermodynamic gradients, namely of temperaturet, (fq=-?t/t2), and of electrochemical potential , (fe=?/t): jq=lqqfq+lqefe and je=leqfq+leefe.lars onsager 2, in work that led to his nobel prize in chemistry, and later hendrik casimir 3, showed that the principle of microscopic time-reversibility means that lqe=leq, while breaking time-reversibility, say by applying a magnetic field b, yields the general onsager-casimir relation lqe(b)=leq(-b).in the case that the system is microscopically reversible (b=0), the efficiency of the thermoelectric steady-state engine depends on the so-called figure of merit 4, zt=(s2/)t=leq2/detl. zt becomes larger the more singular the matrix l becomes, and in the limit zt approaches infinity, one attains the carnot efficiency. the only condition imposed by the second law of thermodynamics is that the entropy produced by the system be greater than or equal to zero, which simply implies the positivity of the matrix l, or, positivity ofzt. optimizing zt is a significant challenge in material science 5. so far, practical values of zt1 are still too low for thermoelectric technology to compete with cycle-based engines or refrigerators. notable exceptions are nanoscale devices 6 and heating or cooling devices where a specialized function, and not efficiency, is most important.in view of these practical limitations, benenti et al. 7 broke new ground by investigating bounds on thermoelectric efficiency in models where microscopic time-reversibility was broken (nonzero b). they derived a general expression for the thermoelectric efficiency in terms of two dimensionless parameters, the generalized figure of merit, y=lqeleq/(detl), and the asymmetry parameter, x=leq/lqe. considering only limitations imposed by the second law, they observed that a range of x and y values were possible. significantly, they observed that it was possible to obtain carnots efficiency for any value of asymmetry|x|1. however, it remained unclear if their model contained all of the ingredients necessary to be considered realistic.in their new work, brandner et al. add this missing ingredient by focusing on a simple steady-state model with three terminalsor contactsto heat and charge reservoirs (fig. 1). in their model, only two of the terminals are connected to a source and drain of heat and electric charge, whereas the third terminal is merely a probe that does not, on average, exchange particles (in this case, charge) and energy with the environment. the reason that adding this third terminal is necessary is that the onsager matrix for the simplest, two-terminal devices with noninteracting particles is always strictly symmetric (x=1), even in the presence of magnetic fields. the minimal model that can break this symmetry has to have three terminals. brandner et al. essentially used a mathematical trick in which they re-parametrized the onsager matrix in terms of a new matrix k, the positivity of which is now a consequence of the law of conservation of charge. the requirement that k be positive puts a constraint on the matrix elements of the onsager matrix that is much stronger than the requirement from the second law and allows for no obvious anomalies, and means the efficiency cannot be as high as benenti et al. predicted. strong evidence that brandner et al.s work will apply to real world models comes from ref. 8, in which the authors numerically calculated the efficiency of a specific, model device and found the same value that brandner et al. now find as an upper bound.brandner et al.s result is not completely bad news about the efficiency of thermoelectric devices. systems with microscopic reversible dynamics can be shown to have a maximum theoretical efficiency (the curzon-ahlborn limit 9) of 1/2c if they operate at maximal output power. (strictly speaking, this limits only holds when the difference of temperatures between hot and cold reservoirs is small with respect to absolute temperatures 10.) using similar assumptions, brandner et al. show that switching on a magnetic field or other vector potential, increases the optimal efficiency of their device at maximum power output to 4/7c. this fundamental result has potentially interesting practical implications for developing small-scale heat engines or ref