crazy. I remember we got a set in the late 80's. It was pretty cool, but those little spheres kinda felt like they were going to fall apart all the time, even with the little plastic bits holding them together. Also, since we only had the beginner set, but instructions for every model possible with all sets were included, we could see all the cool stuff we couldn't build because we had the small set, which was frustrating.
Fastech was also pretty cool, I guess it was sort of like a cheap all-plastic version of an erector set, but instead of screws or anything, it had this plastic rivet-type connection system, with its own rivet-tool dealy. You could build a lot of stuff with that.
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yeah, I want to know why somebody modded him down.
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did s/he really think that the post was off-topic, or did s/he just want to say, "holy shit, I just modded down CmdrTaco. I 0wnz j00 t@c0!!!"
Room-Temperature Ultraviolet Nanowire Nanolasers
Michael H. Huang,1 Samuel Mao,2 Henning Feick,3 Haoquan Yan,1 Yiying Wu,1 Hannes Kind,1 Eicke Weber,3 Richard Russo,2 Peidong Yang1,3*
Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.
1 Department of Chemistry, University of California,
2 Environmental Energy Technology Division,
3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
To whom correspondence should be addressed. E-mail: pyang@cchem.berkeley.edu
The interest in developing short-wavelength semiconductor lasers has culminated in the realization of room-temperature green-blue diode laser structures with ZnSe and InxGa1-xN as the active layers (1-3). ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for blue optoelectronic applications, with ultraviolet lasing action being reported (4-6) in disordered particles and thin films. For wide band-gap semiconductor materials, a high carrier concentration is usually required in order to reach an optical gain that is high enough for lasing action in an electron-hole plasma (EHP) process (7). Such an EHP mechanism, which is common for conventional laser diode operation, typically requires high lasing thresholds. As an alternative to an EHP process, excitonic recombination in semiconductors is a more efficient radiative process and can facilitate low-threshold stimulated emission (8, 9). To achieve efficient excitonic laser action at room temperature, the binding energy of the exciton must be much greater than the thermal energy at room temperature (26 meV). In this regard, ZnO is a good candidate because its exciton binding energy is ~60 meV, substantially larger than that of ZnSe (22 meV) and GaN (25 meV).
To further lower the threshold, low-dimensional compound semiconductor nanostructures have been fabricated, in which quantum size effects yield a substantial density of states at the band edges and enhance radiative recombination due to carrier confinement. The use of semiconductor quantum well structures as low-threshold optical gain media represents a sizable advancement in semiconductor laser technology (10). Light emission from semiconductor nanowhiskers has been previously reported in GaAs and GaP systems (11, 12). Stimulated emission and optical gain have also been demonstrated recently in Si and CdSe nanoclusters and their ensembles (13, 14). Here, we demonstrate excitonic lasing action in ZnO nanowires with a threshold of 40 kW/cm2 under optical excitation.
ZnO nanowires were synthesized with a vapor phase transport process via catalyzed epitaxial crystal growth (15). Using Au thin film as the catalysts for nanowire growth, we epitaxially grew the nanowires, which are highly oriented, on the substrate. Selective nanowire growth can be readily achieved by patterning the Au thin film before growth. Typical scanning electron microscopy (SEM) images of nanowire arrays grown on sapphire (110) substrates with patterned Au thin film (Fig. 1) confirm that the ZnO nanowires grow only in the Au-coated areas. The diameters of these wires range from 20 to 150 nm, whereas more than 95% of them have diameters of 70 to 100 nm. The diameter dispersity is due to the inhomogeneous sizes of the Au nanocluster catalysts when the substrate is annealed during the growth process. The lengths of these nanowires can be varied between 2 and 10 m by adjusting the growth time. The capability of patterned nanowire growth allows us to fabricate nanoscale light emitters on the substrate in a controllable fashion.
Fig. 1. (A through E) SEM images of ZnO nanowire arrays grown on sapphire substrates. A top view of the well-faceted hexagonal nanowire tips is shown in (E). (F) High-resolution TEM image of an individual ZnO nanowire showing its growth direction. For the nanowire growth, clean (110) sapphire substrates were coated with a 10 to 35 Å thick layer of Au, with or without using TEM grids as shadow masks (micro contact printing of thiols on Au followed by selective etching has also been used to create the Au pattern). An equal amount of ZnO powder and graphite powder were ground and transferred to an alumina boat. The Au-coated sapphire substrates were typically placed 0.5 to 2.5 cm from the center of the boat. The starting materials and the substrates were then heated up to 880 to 905C in an Ar flow. Zn vapor is generated by carbothermal reduction of ZnO and transported to the substrates where ZnO nanowires grow. The growth generally took place within 2 to 10 min (15). [View Larger Version of this Image (109K GIF file)]
Because of the good epitaxial interface between the (0001) plane of the ZnO nanowire and the (110) plane of the substrate (16), nearly all of the nanowires grow vertically from the substrates (Fig. 1, A through D). The a plane (110) of sapphire is twofold symmetric, whereas the ZnO c plane is sixfold symmetric. They are essentially incommensurate, with the exception that the a axis of ZnO and the c axis of sapphire are related almost exactly by a factor of 4, with a mismatch of less than 0.08% at room temperature. Such a coincidental matchup along the sapphire [0001] direction, along with a strong tendency of ZnO to grow in the c orientation and the incoherence of interfaces in directions other than sapphire [0001], leads to the unique vertical epitaxial growth configuration. The anisotropy of the sapphire's a plane is critical for growing high-quality c-oriented ZnO nanowire arrays.
Hexagon end planes of the nanowires can be clearly identified in the SEM image of the nanowire array (Fig. 1E), providing strong evidence that these nanowires grow along the direction and are indeed well-faceted at both the end and side surfaces. The well-faceted nature of these nanowires will have important implications when they are used as effective laser media. Additional structural characterization of the ZnO nanowires was carried out with transmission electron microscopy (TEM). The high-resolution TEM image of a single-crystalline ZnO nanowire (Fig. 1F) shows that spacing of 2.56 ± 0.05 Å between adjacent lattice planes corresponds to the distance between two (0002) crystal planes, further proving to be the preferred growth direction for the ZnO nanowires. This preferential nanowire growth on the sapphire substrate is also reflected in the x-ray diffraction pattern (Fig. 2). Only (000l) peaks are observed, indicating excellent overall c-axis alignment of these nanowire arrays over a large substrate area.
Fig. 2. X-ray diffraction pattern of ZnO nanowires on a sapphire substrate. Only (000l) peaks are observed, owing to their well-oriented growth configuration. The diffraction pattern is taken on a Siemens Z5000 x-ray diffractometer. a.u., arbitrary units. [View Larger Version of this Image (12K GIF file)]
Photoluminescence spectra of nanowires were measured with a He-Cd laser (325 nm) as an excitation source. Strong near-band-gap edge emission at ~377 nm has been observed (15). In order to explore the possible stimulated emission from these oriented nanowires, the power-dependent emission has been examined. The samples were optically pumped by the fourth harmonic of Nd:yttrium-aluminum-garnet laser (266 nm, 3-ns pulse width) at room temperature. The pump beam was focused on nanowires at an incidence angle 10 to the symmetric axis of the nanowire. Light emission was collected in the direction normal to the end surface plane (along the symmetric axis) of the nanowires. In the absence of any fabricated mirrors, we observed lasing action in these ZnO nanowires during the evolution of the emission spectra with increasing pump power (Fig. 3, A and B). At low excitation intensity, the spectrum consists of a single broad spontaneous emission peak (Fig. 3A) with a full width at half maximum of ~17 nm. This spontaneous emission is 140 meV below the band gap (3.37 eV) and is generally ascribed to the recombination of excitons through an exciton-exciton collision process, where one of the excitons radiatively recombines to generate a photon (4-6). As the pump power increases, the emission peak narrows because of the preferential amplification of frequencies close to the maximum of the gain spectrum. When the excitation intensity exceeds a threshold (~40 kW/cm2), sharp peaks emerge in the emission spectra. The linewidths of these peaks are 50 times smaller than the linewidth of the spontaneous emission peak below the threshold. Above the threshold, the integrated emission intensity increases rapidly with the pump power (Fig. 3B). The narrow linewidth and the rapid increase of emission intensity indicate that stimulated emission takes place in these nanowires. The observed single or multiple sharp peaks represent different lasing modes at wavelengths between 370 and 400 nm. The lasing threshold is quite low in comparison with previously reported values for random lasing (~300 kW/cm2) in disordered particles or thin films (4). These short-wavelength nanowire nanolasers operate at room temperature, and the areal density of these nanolasers readily reaches 1.1 × 1010 cm2.
Fig. 3. (A) Emission spectra from nanowire arrays below (line a) and above (line b and inset) the lasing threshold. The pump power for these spectra are 20, 100, and 150 kW/cm2, respectively. The spectra are offset for easy comparison. (B) Integrated emission intensity from nanowires as a function of optical pumping energy intensity. (C) Schematic illustration of a nanowire as a resonance cavity with two naturally faceted hexagonal end faces acting as reflecting mirrors. Stimulated emission from the nanowires was collected in the direction along the nanowire's end-plane normal (the symmetric axis) with a monochromator (ISA, Edison, New Jersey) combined with a Peltier-cooled charge-coupled device (EG&G, Gaithersburg, Maryland). The 266-nm pump beam was focused to the nanowire array at an angle 10 to the end-plane normal. All experiments were carried out at room temperature. [View Larger Version of this Image (22K GIF file)]
The observation of lasing action in these nanowire arrays without any fabricated mirror prompts us to consider these single-crystalline, well-faceted nanowires as natural resonance cavities (Fig. 3C). It is possible that the giant oscillator strength effect (8), which can occur in high-quality nanowire crystals with dimensions larger than the exciton Bohr radius but smaller than the optical wavelength, enables the excitonic stimulated emission in these nanowire arrays. For II-VI semiconductors, the cleaved edge of the specimen is usually used as a mirror (1-3, 17). For our nanowires, one end is the epitaxial interface between the sapphire and ZnO, whereas the other end is the sharp (0001) plane of the ZnO nanocrystals. Both can serve as good laser cavity mirrors, considering that the refractive indexes for sapphire, ZnO, and air are
1.8, 2.45, and 1, respectively (18). This natural cavity or waveguide formation in nanowires suggests a simple chemical approach to forming a nanowire laser cavity without cleavage and etching. In fact, when multiple lasing modes were observed for these nanowires (Fig. 3A, inset), the observed mode spacing is ~5 nm for ~5-m-long wires, which agrees quantitatively well with the calculated spacing between adjacent resonance frequencies vF = c/2nl (17), where vF is emission mode spacing, c is the speed of light, n is the refractive index, and l is the resonance cavity length.
Furthermore, lifetime measurements (Fig. 4) show that the radiative recombination of the excitons is a superposition of a fast and a slow process with time constants of ~70 and 350 ps, respectively. The luminescence lifetime is mainly determined by the concentration of defects, which trap the electrons and/or holes and eventually cause their nonradiative recombination. Although the exact origin of the luminescence decay remains unclear at this stage, the long lifetime measured for these wires [350 ps, as compared with 200 ps for ZnO thin films (4)] demonstrates the high crystal quality achieved with the nanowire growth process.
Fig. 4. The decay of the luminescence from the ZnO nanowires was studied with a frequency-tripled mode-locked Ti:sapphire laser for pulsed excitation (200-fs pulse length) and a streak camera with picosecond resolution for detection. A good fit (solid line) to the experimental data (dotted line) recorded at room temperature is obtained with a biexponential decay model assuming a fast and a slow process with time constants of ~70 and 350 ps, respectively. The time-resolved spectrum was recorded at an excitation power of 6.39 mW. [View Larger Version of this Image (15K GIF file)]
REFERENCES AND NOTES
1. D. A. Gaul and W. S. Rees Jr., Adv. Mater. 12, 935 (2000) [ISI].
2. M. A. Hasse, J. Qui, J. M. De Puydt, H. Cheng, Appl. Phys. Lett. 59, 1272 (1991) .
3. S. Nakamura, et al., Jpn. J. Appl. Phys. 35, L74 (1996) [ISI].
4. H. Cao, et al., Phys. Rev. Lett. 84, 5584 (2000) [ISI].
5. D. M. Bagnall, et al., Appl. Phys. Lett. 70, 2230 (1997) [ISI].
6. P. Yu, et al., J. Cryst. Growth 184/185, 601 (1998) .
7. C. Klingshirn, J. Cryst. Growth 117, 753 (1992) [ISI].
8. Y. Kayamura, Phys. Rev. B 38, 9797 (1988) [ISI].
9. W. Wegscheider, et al., Phys. Rev. Lett. 71, 4071 (1993) [ISI].
10. D. Mehus and D. Evans, Laser Focus World 31, 117 (1995) [ISI].
11. K. Haraguchi, et al., Appl. Phys. Lett. 60, 745 (1992) [ISI].
12. X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 409, 66 (2001) [ISI] [Medline].
13. V. I. Klimov, et al., Science 290, 314 (2000) [ISI] [Abstract/Full Text].
14. L. Pavesi, L. D. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408, 440 (2000) [ISI] [Medline].
15. M. H. Huang, et al., Adv. Mater. 13, 113 (2001) [ISI].
16. P. Fons, et al., Appl. Phys. Lett. 77, 1801 (2000) [ISI].
17. B. E. A. Saleh, M. C. Teich, Eds., Fundamentals of Photonics (Wiley, New York, 1991).
18. A simple estimation of the possible number of transversal modes that a waveguide can support indicates that our nanowires with diameters between 80 and 120 nm are actually single-mode waveguides for ultraviolet light.
19. This work was supported by the Camille and Henry Dreyfus Foundation, the 3M Corporation, the NSF through a Career Award (DMR-0092086), the U.S. Department of Energy, and the University of California, Berkeley. P.Y. is an Alfred P. Sloan Research Fellow. Work at the Lawrence Berkeley National Laboratory was supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the U.S. Department of Energy under contract DE-AC03-76SF00098. H.K. thanks the Swiss National Science Foundation for financial support. We thank the National Center for Electron Microscopy for the use of their facilities.
2 March 2001; accepted 26 April 2001
10.1126/science.1060367
Include this information when citing this paper.
I'll just have to file for the sayGoodbye() function. What should I file it as..."Universal method by which a computing device indicates the cessation of activities is imminent." that should work.
I actually got the KQ 1-6 (well 7 too but that doesn't work for whatever reason) combo pack. Farily decent stuff, and KQ1 is still as fun as I remember playing it back in 1990. (Gotta love the computer speaker music). I'll have to get the LSL and maybe space quest games too.
They're also good because I can play them really fast on my Pentium 166 mHz system...sigh
because there are many different types of IP: patents, copyrights, trademarks, etc. etc. etc., and these are different things, have been set up for different reasons, and the issues involved are too different to be covered by one blanket term.
That's as I understood it from the older story which I am too lazy to look up.
It's true that it's not exactly the most pleasant way to make a point.
I think my karma's 1, and today I check out slashdot and there are all these funny drop-down boxes next to every post. I click on the boxes and there are the moderation tags. I thought they had changed the moderation system ('cause I read they were beta-testing their new code or whatever, so I figured that might not be beta any more and that they might have changed the moderation system along with that, because how the hell did I get moderation powers?)
So OlympicSponsor, after having moderated, I guess I can say it's not all that cool--I spend too much time here already, and that's just from reading the +3 and above stuff. Now I have to look at stuff lower than that (and I had NO idea the trolls were that bad) and see if anything's worth anybody else's attention. It's a weird responsibility.
It's snowing over Mt. Olympus! I'll be lucky if my flight gets there! We need to get going if we ever want the B2M (business to martian) contract going.
//end bad joke
From the letter on the website
Our firm represents Economic Solutions, Inc. (ESI) in intellectual property matters. On September 26, 2000 we sent you a letter addressing our client's objection to the granting of either ".biz" or ".bus" top level domains (TLDs). It has come to our client's attention, after visiting your website, that several applications for TLDs proposed a TLD of ".biz" or ".ebiz." In addition to objecting to the granting of a ".biz" or ".bus" TLD, our client also objects to ICANN granting a ".ebiz" TLD as it would substantially damage the rights of ESI in their right to market the ".bz" TLD.
As we mentioned in our previous letter, ESI owns exclusive rights to market the ".bz" country code TLD identifier which it contractually obtained from the government of Belize. ESI has subsequently expended considerable money and effort in its preparations and marketing of the ".bz" TLD as a domain name and has filed numerous trademark applications playing off the phonetic pronunciation of "bz" as "BIZ" and the use of ".bz" in connection with conducting business over the Internet.
ICANN's consideration and approval of an application for a ".biz" or ".ebiz" TLD would create confusion among Internet users, interfere with the rights of ESI, and would cause substantial damage to our client's rights and good will for the same reasons as were outlined in our September 26, 2000 letter. Furthermore, approval of a ".biz" or ".ebiz" TLD would be contrary to ICANN's Criteria for Assessing TLD Proposals, dated 15 August 2000, in light of the existing ".bz" TLD owned by the country of Belize and ESI's plan to target and license domain names under the ".bz" registry to businesses.
The introduction of a ".biz" or ".ebiz" TLD would create substantial confusion among Internet users attempting to access domain names registered with either the ".biz," ".ebiz," or ".bz" registries. The present applications for ".biz" and ".ebiz" specify an intent to target and license domain names to businesses. Due to the similar pronunciations and business connotations associated with ".biz," ".ebiz," and ".bz", Internet users would be confused as to which TLD must be used to locate a business related resource.
what, and bz == business? how does that work? They have the right to stop the creation of.biz just because they are squatting all the.bz domain names so that they can get business hits?
Doesn't all this 3rd world country tld squatting (.bz.tv, etc) strike anybody else as 19th cent. style imperialism? (hey, you won't need those, let us take them).
Obi-wan was wise to put a firewall around her but now his failure is complete.
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Yeah, the TI keyboard was pretty cool
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gotta love fctn-=. I forgot about that style of keyboard untill I downloaded an emulator and wondered where the hell the quote marks were. regular keyboard does not translate very well.
Quoth the submitter: "The $12 million price tag puts it a little out of reach for me and thee right now, but just wait 'til they get open-sourced...:-)"
Ok. I'll let you bug-test v0.9. I think I'll wait till release 1.2
Slashdot Mirror
don't you mean harsh?
or is that the victorian way of saying crack whore?
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at least it's not cowboyGneal...
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I was just going there for the pickled haring, the oude jenever, the stroopwaffelen and the speculaas!
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Not that I had much of a life to begin with, anyway.
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Fastech was also pretty cool, I guess it was sort of like a cheap all-plastic version of an erector set, but instead of screws or anything, it had this plastic rivet-type connection system, with its own rivet-tool dealy. You could build a lot of stuff with that.
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did s/he really think that the post was off-topic, or did s/he just want to say, "holy shit, I just modded down CmdrTaco. I 0wnz j00 t@c0!!!"
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Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.
1 Department of Chemistry, University of California,
2 Environmental Energy Technology Division,
3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
To whom correspondence should be addressed. E-mail: pyang@cchem.berkeley.edu
The interest in developing short-wavelength semiconductor lasers has culminated in the realization of room-temperature green-blue diode laser structures with ZnSe and InxGa1-xN as the active layers (1-3). ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for blue optoelectronic applications, with ultraviolet lasing action being reported (4-6) in disordered particles and thin films. For wide band-gap semiconductor materials, a high carrier concentration is usually required in order to reach an optical gain that is high enough for lasing action in an electron-hole plasma (EHP) process (7). Such an EHP mechanism, which is common for conventional laser diode operation, typically requires high lasing thresholds. As an alternative to an EHP process, excitonic recombination in semiconductors is a more efficient radiative process and can facilitate low-threshold stimulated emission (8, 9). To achieve efficient excitonic laser action at room temperature, the binding energy of the exciton must be much greater than the thermal energy at room temperature (26 meV). In this regard, ZnO is a good candidate because its exciton binding energy is ~60 meV, substantially larger than that of ZnSe (22 meV) and GaN (25 meV). To further lower the threshold, low-dimensional compound semiconductor nanostructures have been fabricated, in which quantum size effects yield a substantial density of states at the band edges and enhance radiative recombination due to carrier confinement. The use of semiconductor quantum well structures as low-threshold optical gain media represents a sizable advancement in semiconductor laser technology (10). Light emission from semiconductor nanowhiskers has been previously reported in GaAs and GaP systems (11, 12). Stimulated emission and optical gain have also been demonstrated recently in Si and CdSe nanoclusters and their ensembles (13, 14). Here, we demonstrate excitonic lasing action in ZnO nanowires with a threshold of 40 kW/cm2 under optical excitation.
ZnO nanowires were synthesized with a vapor phase transport process via catalyzed epitaxial crystal growth (15). Using Au thin film as the catalysts for nanowire growth, we epitaxially grew the nanowires, which are highly oriented, on the substrate. Selective nanowire growth can be readily achieved by patterning the Au thin film before growth. Typical scanning electron microscopy (SEM) images of nanowire arrays grown on sapphire (110) substrates with patterned Au thin film (Fig. 1) confirm that the ZnO nanowires grow only in the Au-coated areas. The diameters of these wires range from 20 to 150 nm, whereas more than 95% of them have diameters of 70 to 100 nm. The diameter dispersity is due to the inhomogeneous sizes of the Au nanocluster catalysts when the substrate is annealed during the growth process. The lengths of these nanowires can be varied between 2 and 10 m by adjusting the growth time. The capability of patterned nanowire growth allows us to fabricate nanoscale light emitters on the substrate in a controllable fashion.
Fig. 1. (A through E) SEM images of ZnO nanowire arrays grown on sapphire substrates. A top view of the well-faceted hexagonal nanowire tips is shown in (E). (F) High-resolution TEM image of an individual ZnO nanowire showing its growth direction. For the nanowire growth, clean (110) sapphire substrates were coated with a 10 to 35 Å thick layer of Au, with or without using TEM grids as shadow masks (micro contact printing of thiols on Au followed by selective etching has also been used to create the Au pattern). An equal amount of ZnO powder and graphite powder were ground and transferred to an alumina boat. The Au-coated sapphire substrates were typically placed 0.5 to 2.5 cm from the center of the boat. The starting materials and the substrates were then heated up to 880 to 905C in an Ar flow. Zn vapor is generated by carbothermal reduction of ZnO and transported to the substrates where ZnO nanowires grow. The growth generally took place within 2 to 10 min (15). [View Larger Version of this Image (109K GIF file)]
Because of the good epitaxial interface between the (0001) plane of the ZnO nanowire and the (110) plane of the substrate (16), nearly all of the nanowires grow vertically from the substrates (Fig. 1, A through D). The a plane (110) of sapphire is twofold symmetric, whereas the ZnO c plane is sixfold symmetric. They are essentially incommensurate, with the exception that the a axis of ZnO and the c axis of sapphire are related almost exactly by a factor of 4, with a mismatch of less than 0.08% at room temperature. Such a coincidental matchup along the sapphire [0001] direction, along with a strong tendency of ZnO to grow in the c orientation and the incoherence of interfaces in directions other than sapphire [0001], leads to the unique vertical epitaxial growth configuration. The anisotropy of the sapphire's a plane is critical for growing high-quality c-oriented ZnO nanowire arrays.
Hexagon end planes of the nanowires can be clearly identified in the SEM image of the nanowire array (Fig. 1E), providing strong evidence that these nanowires grow along the direction and are indeed well-faceted at both the end and side surfaces. The well-faceted nature of these nanowires will have important implications when they are used as effective laser media. Additional structural characterization of the ZnO nanowires was carried out with transmission electron microscopy (TEM). The high-resolution TEM image of a single-crystalline ZnO nanowire (Fig. 1F) shows that spacing of 2.56 ± 0.05 Å between adjacent lattice planes corresponds to the distance between two (0002) crystal planes, further proving to be the preferred growth direction for the ZnO nanowires. This preferential nanowire growth on the sapphire substrate is also reflected in the x-ray diffraction pattern (Fig. 2). Only (000l) peaks are observed, indicating excellent overall c-axis alignment of these nanowire arrays over a large substrate area.
Fig. 2. X-ray diffraction pattern of ZnO nanowires on a sapphire substrate. Only (000l) peaks are observed, owing to their well-oriented growth configuration. The diffraction pattern is taken on a Siemens Z5000 x-ray diffractometer. a.u., arbitrary units. [View Larger Version of this Image (12K GIF file)]
Photoluminescence spectra of nanowires were measured with a He-Cd laser (325 nm) as an excitation source. Strong near-band-gap edge emission at ~377 nm has been observed (15). In order to explore the possible stimulated emission from these oriented nanowires, the power-dependent emission has been examined. The samples were optically pumped by the fourth harmonic of Nd:yttrium-aluminum-garnet laser (266 nm, 3-ns pulse width) at room temperature. The pump beam was focused on nanowires at an incidence angle 10 to the symmetric axis of the nanowire. Light emission was collected in the direction normal to the end surface plane (along the symmetric axis) of the nanowires. In the absence of any fabricated mirrors, we observed lasing action in these ZnO nanowires during the evolution of the emission spectra with increasing pump power (Fig. 3, A and B). At low excitation intensity, the spectrum consists of a single broad spontaneous emission peak (Fig. 3A) with a full width at half maximum of ~17 nm. This spontaneous emission is 140 meV below the band gap (3.37 eV) and is generally ascribed to the recombination of excitons through an exciton-exciton collision process, where one of the excitons radiatively recombines to generate a photon (4-6). As the pump power increases, the emission peak narrows because of the preferential amplification of frequencies close to the maximum of the gain spectrum. When the excitation intensity exceeds a threshold (~40 kW/cm2), sharp peaks emerge in the emission spectra. The linewidths of these peaks are 50 times smaller than the linewidth of the spontaneous emission peak below the threshold. Above the threshold, the integrated emission intensity increases rapidly with the pump power (Fig. 3B). The narrow linewidth and the rapid increase of emission intensity indicate that stimulated emission takes place in these nanowires. The observed single or multiple sharp peaks represent different lasing modes at wavelengths between 370 and 400 nm. The lasing threshold is quite low in comparison with previously reported values for random lasing (~300 kW/cm2) in disordered particles or thin films (4). These short-wavelength nanowire nanolasers operate at room temperature, and the areal density of these nanolasers readily reaches 1.1 × 1010 cm2.
Fig. 3. (A) Emission spectra from nanowire arrays below (line a) and above (line b and inset) the lasing threshold. The pump power for these spectra are 20, 100, and 150 kW/cm2, respectively. The spectra are offset for easy comparison. (B) Integrated emission intensity from nanowires as a function of optical pumping energy intensity. (C) Schematic illustration of a nanowire as a resonance cavity with two naturally faceted hexagonal end faces acting as reflecting mirrors. Stimulated emission from the nanowires was collected in the direction along the nanowire's end-plane normal (the symmetric axis) with a monochromator (ISA, Edison, New Jersey) combined with a Peltier-cooled charge-coupled device (EG&G, Gaithersburg, Maryland). The 266-nm pump beam was focused to the nanowire array at an angle 10 to the end-plane normal. All experiments were carried out at room temperature. [View Larger Version of this Image (22K GIF file)]
The observation of lasing action in these nanowire arrays without any fabricated mirror prompts us to consider these single-crystalline, well-faceted nanowires as natural resonance cavities (Fig. 3C). It is possible that the giant oscillator strength effect (8), which can occur in high-quality nanowire crystals with dimensions larger than the exciton Bohr radius but smaller than the optical wavelength, enables the excitonic stimulated emission in these nanowire arrays. For II-VI semiconductors, the cleaved edge of the specimen is usually used as a mirror (1-3, 17). For our nanowires, one end is the epitaxial interface between the sapphire and ZnO, whereas the other end is the sharp (0001) plane of the ZnO nanocrystals. Both can serve as good laser cavity mirrors, considering that the refractive indexes for sapphire, ZnO, and air are
1.8, 2.45, and 1, respectively (18). This natural cavity or waveguide formation in nanowires suggests a simple chemical approach to forming a nanowire laser cavity without cleavage and etching. In fact, when multiple lasing modes were observed for these nanowires (Fig. 3A, inset), the observed mode spacing is ~5 nm for ~5-m-long wires, which agrees quantitatively well with the calculated spacing between adjacent resonance frequencies vF = c/2nl (17), where vF is emission mode spacing, c is the speed of light, n is the refractive index, and l is the resonance cavity length.
Furthermore, lifetime measurements (Fig. 4) show that the radiative recombination of the excitons is a superposition of a fast and a slow process with time constants of ~70 and 350 ps, respectively. The luminescence lifetime is mainly determined by the concentration of defects, which trap the electrons and/or holes and eventually cause their nonradiative recombination. Although the exact origin of the luminescence decay remains unclear at this stage, the long lifetime measured for these wires [350 ps, as compared with 200 ps for ZnO thin films (4)] demonstrates the high crystal quality achieved with the nanowire growth process.
Fig. 4. The decay of the luminescence from the ZnO nanowires was studied with a frequency-tripled mode-locked Ti:sapphire laser for pulsed excitation (200-fs pulse length) and a streak camera with picosecond resolution for detection. A good fit (solid line) to the experimental data (dotted line) recorded at room temperature is obtained with a biexponential decay model assuming a fast and a slow process with time constants of ~70 and 350 ps, respectively. The time-resolved spectrum was recorded at an excitation power of 6.39 mW. [View Larger Version of this Image (15K GIF file)]
REFERENCES AND NOTES
1. D. A. Gaul and W. S. Rees Jr., Adv. Mater. 12, 935 (2000) [ISI].
2. M. A. Hasse, J. Qui, J. M. De Puydt, H. Cheng, Appl. Phys. Lett. 59, 1272 (1991) .
3. S. Nakamura, et al., Jpn. J. Appl. Phys. 35, L74 (1996) [ISI].
4. H. Cao, et al., Phys. Rev. Lett. 84, 5584 (2000) [ISI].
5. D. M. Bagnall, et al., Appl. Phys. Lett. 70, 2230 (1997) [ISI].
6. P. Yu, et al., J. Cryst. Growth 184/185, 601 (1998) .
7. C. Klingshirn, J. Cryst. Growth 117, 753 (1992) [ISI].
8. Y. Kayamura, Phys. Rev. B 38, 9797 (1988) [ISI].
9. W. Wegscheider, et al., Phys. Rev. Lett. 71, 4071 (1993) [ISI].
10. D. Mehus and D. Evans, Laser Focus World 31, 117 (1995) [ISI].
11. K. Haraguchi, et al., Appl. Phys. Lett. 60, 745 (1992) [ISI].
12. X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 409, 66 (2001) [ISI] [Medline].
13. V. I. Klimov, et al., Science 290, 314 (2000) [ISI] [Abstract/Full Text].
14. L. Pavesi, L. D. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408, 440 (2000) [ISI] [Medline].
15. M. H. Huang, et al., Adv. Mater. 13, 113 (2001) [ISI].
16. P. Fons, et al., Appl. Phys. Lett. 77, 1801 (2000) [ISI].
17. B. E. A. Saleh, M. C. Teich, Eds., Fundamentals of Photonics (Wiley, New York, 1991).
18. A simple estimation of the possible number of transversal modes that a waveguide can support indicates that our nanowires with diameters between 80 and 120 nm are actually single-mode waveguides for ultraviolet light.
19. This work was supported by the Camille and Henry Dreyfus Foundation, the 3M Corporation, the NSF through a Career Award (DMR-0092086), the U.S. Department of Energy, and the University of California, Berkeley. P.Y. is an Alfred P. Sloan Research Fellow. Work at the Lawrence Berkeley National Laboratory was supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the U.S. Department of Energy under contract DE-AC03-76SF00098. H.K. thanks the Swiss National Science Foundation for financial support. We thank the National Center for Electron Microscopy for the use of their facilities.
2 March 2001; accepted 26 April 2001 10.1126/science.1060367 Include this information when citing this paper.
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I could have just given them my 1986 Oldsmobile Delta 88, and saved them a few million dollars.
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(I am american, but we don't really have the most wonderful reputation as travellers)
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Me Grimlock want make fun of Slashdot Editor Grammar, but Grimlock no understand grammar.
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I'll just have to file for the sayGoodbye() function. What should I file it as..."Universal method by which a computing device indicates the cessation of activities is imminent." that should work.
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for the field unix care kit...
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and that's really all I remember about that movie
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I actually got the KQ 1-6 (well 7 too but that doesn't work for whatever reason) combo pack. Farily decent stuff, and KQ1 is still as fun as I remember playing it back in 1990. (Gotta love the computer speaker music). I'll have to get the LSL and maybe space quest games too.
They're also good because I can play them really fast on my Pentium 166 mHz system...sigh
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That's as I understood it from the older story which I am too lazy to look up.
It's true that it's not exactly the most pleasant way to make a point.
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foo
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So OlympicSponsor, after having moderated, I guess I can say it's not all that cool--I spend too much time here already, and that's just from reading the +3 and above stuff. Now I have to look at stuff lower than that (and I had NO idea the trolls were that bad) and see if anything's worth anybody else's attention. It's a weird responsibility.
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for the lame-ass one-click shopping patent?
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It's snowing over Mt. Olympus! I'll be lucky if my flight gets there! We need to get going if we ever want the B2M (business to martian) contract going.
//end bad joke
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From the letter on the website
.biz just because they are squatting all the .bz domain names so that they can get business hits?
.tv, etc) strike anybody else as 19th cent. style imperialism? (hey, you won't need those, let us take them).
Our firm represents Economic Solutions, Inc. (ESI) in intellectual property matters. On September 26, 2000 we sent you a letter addressing our client's objection to the granting of either ".biz" or ".bus" top level domains (TLDs). It has come to our client's attention, after visiting your website, that several applications for TLDs proposed a TLD of ".biz" or ".ebiz." In addition to objecting to the granting of a ".biz" or ".bus" TLD, our client also objects to ICANN granting a ".ebiz" TLD as it would substantially damage the rights of ESI in their right to market the ".bz" TLD.
As we mentioned in our previous letter, ESI owns exclusive rights to market the ".bz" country code TLD identifier which it contractually obtained from the government of Belize. ESI has subsequently expended considerable money and effort in its preparations and marketing of the ".bz" TLD as a domain name and has filed numerous trademark applications playing off the phonetic pronunciation of "bz" as "BIZ" and the use of ".bz" in connection with conducting business over the Internet.
ICANN's consideration and approval of an application for a ".biz" or ".ebiz" TLD would create confusion among Internet users, interfere with the rights of ESI, and would cause substantial damage to our client's rights and good will for the same reasons as were outlined in our September 26, 2000 letter. Furthermore, approval of a ".biz" or ".ebiz" TLD would be contrary to ICANN's Criteria for Assessing TLD Proposals, dated 15 August 2000, in light of the existing ".bz" TLD owned by the country of Belize and ESI's plan to target and license domain names under the ".bz" registry to businesses.
The introduction of a ".biz" or ".ebiz" TLD would create substantial confusion among Internet users attempting to access domain names registered with either the ".biz," ".ebiz," or ".bz" registries. The present applications for ".biz" and ".ebiz" specify an intent to target and license domain names to businesses. Due to the similar pronunciations and business connotations associated with ".biz," ".ebiz," and ".bz", Internet users would be confused as to which TLD must be used to locate a business related resource.
what, and bz == business? how does that work? They have the right to stop the creation of
Doesn't all this 3rd world country tld squatting (.bz
end rant.
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gotta love fctn-=. I forgot about that style of keyboard untill I downloaded an emulator and wondered where the hell the quote marks were. regular keyboard does not translate very well.
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I get a little, ahem, frustrated sometimes when the goddamn alien controlers kick my ass every single time.
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way too inflammatory, even if BG3 is a major league asshole
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Ok. I'll let you bug-test v0.9. I think I'll wait till release 1.2
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