Ancient Diets, Ecology, and Extinction of 5-Million-Year-Old Horses from Florida

B. J. MacFadden
1999 Science  
ative gate bias increases, the Fermi level approaches the delocalized band edge and more traps are filled. At an appropriately high voltage, all of the trap states are filled, and any subsequently injected carriers are free to move with the microscopic mobility associated with carriers in the delocalized band. In devices with a 0.12-m-thick SiO 2 gate insulator, the negative gate bias required to reach such carrier concentration levels was above 70 to 80 V. As shown in Fig. 2 , the mobility
more » ... , the mobility saturates above this gate bias, indicating that all localized states are occupied. By using higher dielectric constant insulators, a sufficient number of carriers to fill all these traps are generated by the field effect at much lower gate voltages, and this is the reason that high mobilities are observed at such low voltages compared with the SiO 2based devices. Several localized trap levels have been reported for thin polycrystalline vapor-deposited films of pentacene at depths ranging from 0.06 to 0.68 eV (23), which could account for the aforementioned localized states. Brown et al. (17) have used an alternative model, based on hopping polaron transport processes, to explain the gate bias dependence in organic semiconductor IGFETs. On a more practical note, all of the steps in the fabrication process of our low operating voltage BZT-pentacene-based devices were performed entirely at RT. Initially, we used sol-gel-deposited BST films as the high gate insulator (19), which required annealing at 400°C, and although they produced excellent low-voltage device characteristics, they were not compatible with transparent plastic substrates. The RT fabricated devices we now report are compatible with transparent plastic substrates. Given their operating characteristics, which are very close to the characteristics of the widely used a-Si:H TFT, they are good candidates for applica-tions involving AMLCD or AMOLED displays on plastic substrates. Several efforts in the past to develop a-Si:H TFT-based liquid crystal displays on plastic have not been successful because of deterioration of the performance of those TFTs when a-Si:H was grown at temperatures below 150°to 200°C. To demonstrate the feasibility of the above idea, we successfully fabricated pentacenebased TFT on very transparent plastic substrates (polycarbonate). The BZT gate insulator was 0.128 m thick. Their performance was similar to devices fabricated on quartz or SiO 2 / Si substrates. Figure 4 shows the characteristics of such a device (W ϭ 1500 m and L ϭ 69.2 m). Mobility was 0.2 cm 2 V Ϫ1 s Ϫ1 , as calculated in the saturation region. Mobility values as high as 0.38 cm 2 V Ϫ1 s Ϫ1 were measured from devices with a W/L ratio of 4. These are the highest reported mobilities from devices fabricated on plastic substrates, and they are obtained at a maximum gate voltage of only 4 V. The highest mobility reported before for organic IGFET on plastic substrates was up to 0.1 cm 2 V Ϫ1 s Ϫ1 , and it required the operating voltage to be modulated between 0 and Ϫ100 V (24). Six sympatric species of 5-million-year-old (late Hemphillian) horses from Florida existed during a time of major global change and extinction in terrestrial ecosystems. Traditionally, these horses were interpreted to have fed on abrasive grasses because of their high-crowned teeth. However, carbon isotopic and tooth microwear data indicate that these horses were not all C 4 grazers but also included mixed feeders and C 3 browsers. The late Hemphillian Florida sister species of the modern genus Equus was principally a browser, unlike the grazing diet of modern equids. Late Hemphillian horse extinctions in Florida involved two grazing and one browsing species. The fossil record of horses (family Equidae) over the past 55 million years is a classic example of macroevolution. In the traditional interpretation, during the Eocene through ear-ly Miocene [ϳ55 to 20 million years ago (Ma)], horses had short-crowned teeth, indicating a browsing diet of leafy and soft plants. After ϳ20 Ma, fossil horses underwent an explosive adaptive radiation resulting in numerous clades with high-crowned (hypsodont) teeth adapted for grazing on abrasive plants, particularly grasses. This connection between observed tooth crown height and interpreted diets of extinct horses is a widely accepted model of long-term adaptation and evolution (1). Fig. 4. Plot of I D versus V D at various V G from an organic IGFET on polycarbonate.
doi:10.1126/science.283.5403.824 pmid:9933161 fatcat:sgyqi4qtavghlpivbgylqqcgrq