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Microcavity-controlled chirality-sorted carbon nanotube film infrared light emitters Shuang Liang, Nan Wei, Ze Ma, Fanglin Wang, Huaping Liu, Sheng Wang, and Lian-Mao Peng ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00856 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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ACS Photonics

Microcavity-controlled chirality-sorted carbon nanotube film infrared light emitters Shuang Liang,† Nan Wei,† Ze Ma,† Fanglin Wang,† Huaping Liu,ζ,ǁ Sheng Wang,†,* Lian-Mao Peng†,‡,* †

Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, and

‡

Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China.

ζ

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, China. ǁ

Collaborative Innovation Center of Quantum Matter, Beijing 100190, China.

*

Address correspondence to [email protected], [email protected]

Abstract: Applied as on-chip infrared (IR) light sources for future nanophotonic circuits and information optoelectronics, light emitters should show the narrow spectral width, strong emission, low onset voltage, and better tunability of light output to an external drive. Here, by utilizing small-diameter (d < 1 nm) chirality-sorted (8,3) and (8,4) carbon nanotube (CNT) films and its charged excitons (trions) electroluminescence (EL), we achieve performance improvements via channel length (Lch) scaling. With a short Lch, the devices can show better emission, and the external EL efficiency ( ) in free space can reach ~ 6×10-4 (that is obtained at the current of ~ 5−8 mA and the voltage of ~ 4−6 V from the 0.5-μm-channel device, and the corresponding current density is ~ 1700−3000 Acm-2). The strong emission at smaller bias makes CNT-based emitters have a wider optoelectronic compatibility with other nanomaterial systems. Furthermore, by an integration of the emitter with a λ/2 optical cavity, the cavitycontrolled well-defined light output can be achieved, with narrow spectral widths at selectable emission windows (e.g., ~ 28 meV at the wavelength of 1180 nm). The results show possible applications of chirality-sorted CNT film light emitters for further on-chip nanophotonic systems.

KEYWORDS: carbon nanotube film, electroluminescence, optical cavity, impact excitation, channel scaling, on-chip light source.

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The extension of low-dimensional semiconductors to integrated optoelectronics has brought great benefits to technical developments and scientific discoveries.1 Nanoscale light emitters with the low onset voltage, bright emission, narrow spectral width, and high sensitivity to a drive are very important for nanophotonic circuits and on-chip optical interconnects.2-7 Carbon nanotubes (CNTs) potentially provide many advantages for applications as nanoscale light emitters. First, it has stable, self-closing lattice structure consisting of strong covalent bonds. The large exciton binding energies of hundreds of meV as a result of electron-hole (e--h+) interactions in tightly confined one-dimensional systems provide the possibility for electrically controllable dipole-like radiation, even coherent source.8,9 Second, unlike the bandgap engineering in conventional semiconductors through composition alloying or size-induced quantum confined effect, the bandgap energies (Eg) of CNTs naturally depend on the different chiral angles and diameters. The first van Hove transitions (E11) of different CNTs can cover a wide wavelength range with specific luminescence peaks,10 making it possible to fabricate multiple-color light sources for nanoscale photonics. Third, low-cost chirality separation methods for high-purity CNTs have been promoted, and highly-integrated nanoelectronic devices have been developed rapidly that pave a pathway for possible on-chip optoelectronic unity. Besides, the advantages of CNTs in doping-free fabrication for the diode-type device,11 and easy local positioning, and pattern etching also provide the technical flexibility. The electroluminescence (EL) from CNTs due to neutral exciton radiation has been achieved through mechanisms such as e--h+ pairs recombination,12-15 thermal excitation,16 and impact exictation.7,17-19 In recent years, emergence of charged excitons (trions) in CNTs attracts

many attentions. Binding energy of the trion is ~ 60/d + 70/d2 meV (d in nm), corresponding to


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the energy separation between the bright singlet excitons and the trions.20-25 For small-diameter CNTs (d < 1 nm), the binding energy is more than one hundred meV that can provide a good stability for the existence of trions at room temperature (or higher temperature). On the one hand, the optically bright trions with non-zero spins are sensitive to external magnetic field, and the additional spin in the three-particle system can be considered as a new degree of freedom for enriching conventionally optoelectronic devices.26 On the other hand, in small-diameter CNTs, a

large energy splitting (∆ ≈ 70/d2) can exist between radiative singlet and non-radiative triplet energy levels. The dark triplet excitons occupy a higher proportion in the numbers of excitons due to a factor of [1+3exp(∆/kBT)], thus obviously reducing the luminescence efficiency of CNTs.12 With lightening the dark triplet excitons via ultrafast charging process of neutral excitons, the luminous efficiency can be expected to improve.20-23 In this research, through the use of different electrodes and analysis of EL mechanism, we show the impact-excitation-induced trion emissions from chirality-sorted (8,3) and (8,4) CNT films. With channel length (Lch) scaling, we demonstrate that tandem impact excitations along a current flow path between the electrodes have an evident influence on EL. The optimized

external EL efficiency ( ) from the film devices with Lch = 0.5 μm can reach ~ 6 × 10-4 at the input current of ~ 5−8 mA and driving voltage of ~ 4−6 V (the corresponding current density is ~ 1700−3000 Acm-2). Furthermore, in view of the fact that the aggregate of CNTs usually leads to a very broad (or uncertain) luminescence spectrum due to the different Eg, carrier Auger effect, or Fermi-Dirac potential redistribution,27,28 the cavity-controlled emitter was further fabricated to output a well-defined emission. The coupling mode is designed by varying the optical length between the counter reflectors, and can output the radiation with an obviously improved spectral width at a coupling wavelength. To our knowledge, by utilizing CNT films as active materials, this is a very positive improvement for strong emission with an excellent spectral profile. The integration method can also be used for other CNT films, and it can provide a possibility for the application as on-chip light source in a highly integrated photonic or optoelectronic platform. Overall, the following distinctive advantages can be achieved. i) Better and specific emission properties from CNT film devices, including a high-sensitivity dependence of emission intensity on driving voltage, and a strong output at a small bias. The EL from present CNTs can cover the important communication wavebands (in the range of ~ 1200−1550 nm). ii) Selectable coupling windows. The output can be selected through tuning the resonant wavelength of the optical


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cavity within EL from films. A narrower spectrum can be achieved through resonant feedback effect. iii) Stable emission performances without uncertain spectral profiles induced by random surroundings. With the advantages of strong emission, small driving bias, definite emission wavelength and narrow spectral width, cavity-integrated chirality-sorted CNT film emitters based on electrically-induced impact excitation mechanism are expected to satisfy further chipintegrated information optoelectronics. RESULTS AND DISCUSSION A structural cross-section of the typical device is shown in Figure 1a. From bottom to top, Si/SiO2 wafer was used as the supporting substrate, 100-nm-thick Ag layer was the bottom reflector, the SiO2 and the HfO2 layers were the bottom half-space of the cavity, and the polymethylmethacrylate (PMMA) layer was used as the top half-space of the cavity. The chirality-sorted CNTs separated by gel chromatography method mainly contain (8,3) and (8,4)

tubes,29 with an average tube length of ~ 0.7−1.5 μm. For more details, see Figure S1 in Supporting Information. The CNT films were deposited on HfO2 layer and were contacted

electrically by using symmetric Ti (0.5 nm)/Pd (30 nm) electrodes with contact length Lc = 1 μm (for fabrication, see Methods). The Pd electrode mainly aligns with the valence band of CNTs, and makes it easier to inject holes into channel regions.30 To identify intrinsic emission properties, free-space EL and photoluminescence (PL) spectra were first measured (Figure 1b). It can be observed that obvious redshifts exist between PL and EL spectra, ~ 210 (205−230) meV from (8,3) CNTs and ~ 140 (135−155) meV from (8,4) CNTs. Such energy separations are consistent with previous reports on the trion emission (see Figure S2).20-25 For optical excitation, an equal number of e- and h+ can usually be created by incoming photons, and subsequently they form the neutral excitons along the intrinsic (undoped) CNTs. For EL measurement (Figure 1c), the source and the back-gate electrodes were grounded. As the drain electrode was biased from

1.8 V to 3.4 V on the representative device with Lch = 1 μm, the emission intensity shows an exponential dependence on Vds, but the source-drain current (Ids) has an approximately linear trend. For measurements at larger Vds, see Figure S3. The dependence of integral EL intensity on Vds is investigated, using Eq. 1 with physical parameters of b = 5×10-2 eV, α = 4.57×10-3 nm-1,

Eth ≈ Eg, and Feff = Vds. Eth is the impact excitation threshold energy, Feff is the band bending field for an excitation site, and b is other factors such as thermal effect. The optical phonon


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scattering length (≈ 14 × d) is ~ 13 nm. The fit trend indicates that the emission intensity is proportional to impact excitation rate (the AL is a linear factor).7,17,18 ∝ exp

−

!

% (1'

+ #! $

During the EL, the unipolar carriers (holes) can be mainly injected into channel regions with symmetric Ti/Pd electrodes, and impact-excitation-induced excitons can be rapidly charged to form trions (the lowest excitonic state in CNTs). In fact, the generation rate of excitons (excitons per hole) by impact excitation is relatively low,7,17 and abundant unipolar carriers existing around an excitation site can be used as dopants. The ultrafast charging process of neutral excitons (less than several picoseconds) provides a foundation for forming trions.31-34 The relatively low Eth of small-bandgap tubes makes the EL from (8,4) CNTs more or less stronger than that from (8,3) CNTs. Further, the EL spectrum from devices with symmetric Ti/Pd electrodes is also compared with the result from devices with asymmetric electrodes (Figure 1d). With asymmetric Ti/AuTi/Pd electrodes, e- and h+ can be injected into channel regions, respectively. Then, the neutral exciton emissions (labeled by gray lines) from (8,3) and (8,4) CNTs can be observed at the small bias, as a result of e--h+ pair recombination.35 Usually, exciton energy transfer (EET) can occur among chirality-mixed CNTs. That means excitons created in smaller-diameter (larger-bandgap) CNTs can partly decay into largerdiameter (smaller-bandgap) CNTs, leading to a changed spectral profile such as peak positions and spectral widths.36 Previous study, with a high-resolution optical excitation technique on pairs of semiconducting CNTs, has observed the short-range effect that is affected by some nonradiative relaxation pathways and low luminescence yield in CNTs.37,38 In this research, the EET will refer to the process that the EL from larger-bandgap CNTs optically excites smaller-bandgap CNTs, and the photon-absorption and re-radiation (i.e., the PL process) in smaller-bandgap CNTs will determine the EET intensity. For present film, we analyzed the trion peak intensity (at 1180 nm) of (8,3) CNTs I(8,3) versus that (at 1340 nm) of (8,4) CNTs I(8,4) within a bias range (e.g., 1.8−3.4 V) from the 1-μm-channel device (Figure 1e). The linear ratio of I(8,3)/I(8,4) can be confirmed without obviously changed spectral profile, and a statistic result from 12 devices


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further shows the stable emission (inset). For the EET in present CNTs, two low-efficiency steps are involved, i.e., the photon-absorption and re-radiation processes. The small absorption cross section (i.e., most of the photons will pass through the CNT without effective absorption) will weaken photon absorption in smaller-bandgap CNT, thus strongly affecting the following energy transfer process. Besides, assuming one photon (from EL of larger-bandgap CNTs) can be captured by smaller-bandgap CNT, the re-radiation efficiency of the photon is still very low (in general, the luminescence efficiency is < 1%), determined by the quality of the used CNTs, tube length, and the surroundings. The local impact-excitation-induced EL also largely weakens the possibility of the large-scale EET process along whole tubes. Thus, the EET cannot obviously affect the final EL spectra induced by stronger carrier injections. To achieve electrically-induced impact excitation, the injected carriers need to obtain enough kinetic energy through acceleration in the electrostatic field and excite electrons from valence band to conduction band (or radiative energy levels). The situation has been previously observed in suspended single-tube device supported on SiO2, where a sharp band bending derived from a change in dielectric environments at the suspended/supported interface creates an excitation site.7 The impact excitation sites can also be formed through defects or tube-tube

junctions. In tightly-confined one-dimensional CNT, a longer (or lower threshold electric field, ( ≈ Eg/e ) and a larger exciton binding energy all provide the foundations for impact-

excitation-induced excitonic radiation, which is a more probable process than that in bulk materials. The dependence of EL on different Lch is shown in Figure 2a and the Vo is defined as the voltage for detectable emission from a device with a specific Lch. The lowest Vo of ~ 1.2 V is from the 0.5-)m-channel device, and the Vo cannot be further decreased by shorter channels (Lch

≤ 0.5 )m). When Lch increases from 0.5 )m to 7 )m, the Vo indeed increases from ~ 1.2 V to ~ 8 V. Besides, at a constant bias (~ 8 V), emission intensity will decrease by ~ 5 orders of magnitude with increasing Lch. To further discuss this mechanism, we will investigate the dependence of EL property on tandem excitation sites, with Lch scaling. When multiple current flow paths exist between two electrodes, the parallel excitation sites at different paths will linearly increase total EL intensity (they are included in the AL factor). Thus, a typical current flow path is discussed, where tandem

excitation sites can be included with increased Lch. For a short Lch (e.g., Lch = 0.5 μm), the single


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excitation site may be mainly included in a path (the numbers of impact excitation sites in a current flow path can be estimated through the threshold or detectable voltage), and the corresponding EL process is shown in the top panel of Figure 2b. For a longer Lch (bottom panel), a representative case with two tandem excitation sites along a current flow path is illustrated. The red arrows depict that holes obtain kinetic energy through the acceleration in electrostatic field, and dashed lines show the impact excitation, charging and trion radiation processes. Dependences of emission intensity on Vds from different devices are shown in Figure 2c, and the short-channel device shows a sharper variation in emission intensity within a smaller bias range. For example, for 0.5-)m-channel device, a small bias change of ~ 2 V can lead to an intensity variation of three orders of magnitude. But for 5-)m-channel devices, the same trend needs a

wider bias change of > 10 V. For short-channel devices (Lch = 0.5 μm), with parameters of L= 4×108, b = 5×10-2 (eV), and α = 4.1×10-3 nm-1, the experimental data can be fitted by using Eq. 1.

The trend of emission intensity, threshold energy field (Eg/e ≈ 1 MV/cm), and band bending

field ( Vds = 4.1×10-3 nm-1 × 1.2 V ≈ 0.05 MV/cm) for impact excitation are similar with

previous reports based on a single-tube device, where a single excitation site was constructed.7,17,18 In fact, the Eq. 1 can be simplified into electric field form, exp(-( /(+, ), with

!

> # ! and ( = Eg/e (≈ 1 MV/cm that is consistence with a predication of ~

0.9// ! MV/cm) and (+, = Vds ≈ 0.05 MV/cm. The ( is threshold electric field needed for

impact excitation, and (+, is electrostatic interface field for a band bending. For each excitation

site, the specific ( (i.e., Eth > radiation energy Ephoton) and (+, (~ 0.05 MV/cm) are all needed

to produce specific excitons. When the energy conditions are satisfied, the exponential equation will give the rate of impact excitation. For each excitation site, the impact excitation cannot take

place for Eth below Ephoton or band bending field below (+, . With increasing Lch, more excitation sites can be included in a current flow path between source and drain, but the energy conditions for EL (at each excitation site) are invariable, thus tandem impact excitations should play a role.

Furthermore, for other Lch, the L and the Feff in a current flow path can be estimated through the factor of Vo/1.2 (Vo/1.2 can be considered as the number of impact excitation sites in a current flow path for a specific Lch). For example, parameters of (3.3/1.2)L (an increase in the numbers

of excitation sites) and Feff = α(1.2/3.3)Vds (an effective electric field at an excitation site) can be adopted for 2-)m-channel devices. The simulation results based on Eq. 1 (labeled by solid lines in Figure 2c) are consistent with the experimental data. Note that EL processes from different


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excitation sites at different current paths are not exactly equivalent or synchronous. At a larger bias, some dynamic variations (e.g., some varying or increased current flow branches) maybe exist in some films, which can be induced by varying surroundings (e.g., the variations of the remaining surfactant at high temperature). Such variations (at larger bias) can have stronger influences on EL because of the exponential relation, leading some deviations from Eq. 1 (e.g., the case at shorter Lch). The deviation indeed depends on actual situations in some films, and could be introduced by further superposition of piecewise exponential functions. However, if the EL is dominated by e--h+ pairs recombination, the light emitter can still show a strong emission

at smaller Vds with further shortening Lch (Lch ≤ 0.5 μm) because of the more effective injection

of e- and h+, which cannot be observed in experiments. The localized luminescence process of impact excitation in an one-dimensional structure can be expected to create brighter emission.7,9,17,39,40 With increasing Vds from Vo, the exciton

generation rate (or emission intensity) will be enhanced and (photons per injected hole) can

show an increase (see inset in Figure 2d or Figure S3). At a specific Vds (or current density) range,

the increased trend of will be moderate. With current density of ~ 1700−3000 Acm-2 and

voltage of ~ 4−6 V from a 0.5-μm-channel device (Figure 2d), is ~ 4×10-4−6×10-4 without a significant dependence on Lch (Note1 in Supporting Information). Compared with the reported

from other high-quality nanomaterials (e.g., is ~ 1×10-4 at an input current of hundreds

of nA),12,41 the emission intensity has been improved obviously. The operation at a higher current density (for stronger emission) can still be expected, if the oxidative damage at high temperature can be avoided. For impact excitation, the hot carriers will transfer accumulated kinetic energies to crystal lattices by random collisions, thus thermal effect is further investigated. To reduce the oxidative damage of CNTs at high temperature, a ~ 30-nm-thick HfO2 layer was grown on devices. With increasing Vds from 0 V to 18 V in a 1-μm-channel device, the temperature T within the channel region can be estimated based on the peak position of G-mode of Raman spectra (Figure 3a).42 Through the use of a PMMA coating layer on the device (with a decomposition temperature Td of ~ 570 K), the T at ~ 570 K within channel regions can also be actually calibrated (i.e., Td corresponds to ~ 10−12 V, labeled by violet arrows in the top panel of Figure 3b). For devices

with other Lch, the corresponding Vds for Td can be ~ 6−7 V for Lch = 0.5 μm, and ~ 17−20 V for


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Lch = 2 μm. The analysis from and T indicates that the factor of ( '! is larger than

(kBT'! for present Vds range (bottom panel of Figure 3b), where Feff is α(1.2/Vo)Vds with Vo = 1.8 V for 1-μm-channel devices. Thus, the blackbody background radiation from thermal effect is not dominant, which is also confirmed by all EL spectra. The increased T will cause some adverse effects on EL, e.g., the possible dissolution of trions into triplet excitons (the energy

separation between the triplet excitons and the trions is ~ 60/d meV) or spectral overlaps derived

from different CNTs. For applications in chip-integrated nanophotonic circuits, the specific information stored in signals should be outputted, thus an improvement is further needed. The integration of nanoscale CNTs with photonic devices is a key step to achieving further applications in photonic circuits or information optoelectronics. Several photonic nanocavities (e.g., photonic crystals or microring resonators with a high quality factor [Q factor] of > 1000) have been attempted, with effective coupling and narrower linewidth via the optical or the electrical excitation.6,43-48,50 Here, the optoelectronic integration of the CNTs with Fabry-Perot (F-P) cavity is achieved, the fabrication process of the cavity can be compatible with highintegration electronics. The directed output can be realized with a narrower linewidth, and the configuration can be expected to achieve nanoscale optoelectronic unity in the vertical direction. The EL from chirality-sorted CNT films can give specific radiation. To realize effective coupling between the radiation from active materials and the allowed mode of the cavity, the thickness of

the dielectric layer will be designed based on the refractive indices (=', ensuring that the active

region is located nearly at the center of the allowed mode. With =>+?! = 1.35, =@?! = 1.89, and

=ABBC = 1.42, the thickness of each layer can be considered for a specific resonant output ( D '.

As an example, when on-axis D is ~ 1285 nm (shown by the orange line in the bottom panel of Figure 4a), the thicknesses of each layer can be designed, i.e., 100 nm of Ag bottom reflector,

140 nm of SiO2 layer, 30 nm of HfO2 layer, 175 nm of PMMA layer, and 30 nm of Au top

reflector. The designed D is within the EL range of the used CNTs, and the coupling is expected after coating the Au reflector. Unlike the free-space spectrum (labeled by the gray line in the top

panel), the output EL from the cavity-integrated device with Lch = 0.5 μm can match with the basic mode of the cavity, represented by the peak position, the spectral width, and the line shape. Compared with the full width at half maximum (FWHM) of ~ 295 nm (~ 220 meV) in free space, a much narrower spectral FWHM of ~ 33 nm (~ 26 meV) can be obtained from cavity


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effect. Meanwhile, the devices with on-axis D = 1180, 1339 and 1443 nm are also fabricated with the FWHM of 30−35 nm, showing tunable outputs due to different cavity lengths. The finite-element simulation of E-field distribution is performed (Figure 4b), and the half-wave optical mode within the cavity can be established. The directed emission which is desirable for high-efficiency on-chip light coupling is shown. For the typical cavity ( D ≈ 1285 nm), the

dependence of on-axis resonance output on Au thickness is investigated (Figure 4c). With increasing the thickness from 15 nm to 30 nm, a wavelength blueshift from 1320 nm to 1285 nm can be observed (inset), which is associated with a shorter optical length due to the gradually increased reflectivity of Au reflector. The narrowing of spectral FWHM is associated with the improved resonance effect of the cavity, thus the Q factor (Q = D ⁄FWHM) will increase. With 30-nm-thick Au layer, the Q factor can reach ~ 40 for present cavities. The emission wavelength

as a function of viewing angle (J) is also shown (Figure 4d). With increasing J from 0° to 30°, a

moderate trend of off-axis emission from 1285 nm to 1256 nm can be observed, with decreased emission intensity. The behavior can be explained by a constant on-axis resonance energy, which is a component of the off-axis radiation mode.49 The off-axis emission can be further suppressed by using a narrower spectrum from single-chirality CNTs due to detuning effect. For on-axis emission, if the cavity mode can overlap with either one of (8,3) or (8,4) trion peaks, the EL intensity can be obviously improved due to a stronger coupling. Figure 4e shows that the cavity-

integrated device selectively outputs the trion emission ( D = 1180 nm) from (8,3) CNTs with a

narrowed FWHM of ~ 30 nm (~ 28 meV), and the estimated by peak intensity is ~ 5×10-5.

The decreased can be associated with the light absorption in the Au layer. For the present F-

P cavity, with increasing the thickness of top Au reflector, the Q factor of the cavity can be significantly improved; however, the Au layer can also absorb the parts of the output light. With increasing Vds (Figure 4f), the peak intensity will exponentially increase, while the optimized emission can be kept. The stable, well-defined, and exponential dependence of EL intensity on driving voltage can be expected to have applications in chip-integrated photonic circuits and information optoelectronics. CONCLUSIONS In summary, we have shown the trion emission from small-diameter CNTs, with electricallyinduced impact excitation mechanism. We proposed that the numbers of excitation sites along a


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current flow path within the channel region have a significant influence on EL emission, thus the emitter can be significantly optimized via Lch scaling. With a proper Lch, the emitter can show better emission at a small driving voltage. Furthermore, an integration of λ/2 optical cavity with the optimized emitter is made, and a significantly improved spectrum is achieved. The result implies that cavity-integrated chirality-sorted CNT films can be considered as a strong on-chip light source for prospective optical interconnects or hybrid optoelectronic systems. MATERIALS AND METHODS Materials. The chirality-sorted (8,3) and (8,4) CNTs, grown by high-pressure carbon monoxide process (HiPco), were separated by gel chromatography method and were dissolved into 2% lauryl sodium sulfate (SDS) solution. To prepare CNT thin film, the substrate (HfO2 layer) was decorated with a single-layer (3-aminopropyl) triethoxysilane (APTES), and several drops of CNT solution were dropped on HfO2 layer (the deposition of CNTs was kept for 3 hours at room temperature and normal pressure). Then, the sample was immersed into deionized water (for 12 hours) and dried by nitrogen gas. Finally, the sample was stored in vacuum before device fabrication. Careful measurements by using PL technique confirmed the same components between the CNT solution and the films. Device Fabrication Methods. For device fabrication, the Si wafer with a 300-nm-thick thermal

oxide layer was used as the supporting substrate. An example with D = 1285 nm is illustrated. First, a 100-nm-thick Ag reflector and a 140-nm-thick SiO2 layer were deposited with ebeam evaporation. Second, a 30-nm-thick HfO2 layer was grown by atomic layer deposition (ALD) at low temperature (90℃). The two oxide dielectric layers (i.e., SiO2 and HfO2) were used as the bottom half-space of the λ/2 optical cavity. Then, CNTs were deposited on the HfO2 layer

by drop-coating method. The interdigitated (array) electrodes were fabricated with a total device size of ~ 15 μm × 20 μm and Lc = 1 μm. Raith150 e-beam lithography (EBL) was used to pattern

the device structure on 150-nm-thick PMMA e-beam resist. Ti (0.5 nm)/Pd (30 nm) or Ti (0.5 nm)/Au (30 nm) electrodes were deposited with e-beam evaporation, followed by the lift-off in acetone. Redundant CNT films outside the channel region were etched by inductively coupled plasma (ICP) method. Finally, a 175-nm-thick PMMA dielectric layer fabricated by spin-coating method was used as the top half-space of the optical cavity, and a 30-nm-thick Au reflector was deposited with e-beam evaporation.


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Measurements. PL and Raman spectra of the CNT films were measured by using an UV-NIR HR800 (Jobin Yvon/Horiba) confocal system, with the excitation wavelength of 633 or 785 nm. The optical system is equipped with a liquid-nitrogen-cooled InGaAs detector operating in the wavelength of 800−1700 nm. Samples were placed on a three-dimension motorized translation stage. To confirm the chirality-sorted CNTs, PLE mapping was performed through a spectrofluorometer (HORIBA, NanoLog) with 5-nm resolution step. During EL measurement, the Leica microscope (50×, NA = 0.55) was focused on devices, and the source-drain voltage and current were measured by using Keithley4200 Semiconductor Characterization System under normal atmospheric condition.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: PLE and absorption spectra for chirality identification of CNTs, CNT morphology by atomic force microscope (AFM), PL spectra of the trion with chemical doping, a typical dependence of EL on the bias, and the method for . This material is available free of charge via the Internet at http://pubs.acs.org Conflict of Interest: The authors declare no competing financial interest. ACKNOWLEDGMENTS. This work was supported by the National Key Research & Development Program (Grant No. 2016YFA0201902) and National Science Foundation of China (Grant Nos. 61370009, 61621061). S. L. acknowledges the support by China Postdoctoral Science Foundation (2014M550559). H.L. acknowledges the support by the recruitment program of global youth experts and the "100 talents project" of CAS. REFERENCES (1) Avouris, P.; Freitag, M.; Perebeinos, V. Carbon-Nanotube Photonics and Optoelectronics. Nat. Photonics 2008, 2, 341−350. (2) Shacham, A.; Bergman, K.; Carloni, L. P. Photonic Networks-on-Chip for Future Generations of Chip Multiprocessors. IEEE Trans. Comput. 2008, 57, 1246−1260.


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ACS Photonics

Wavelength (µm) 1.5 1.4 1.3 1.2 1.1 1 (8,4) (8,3)

EL

PL

140 meV

c 5

×10

210 meV

(8,4) (8,3)

2.5

120 Fit curve

2.0

Measured data Ids

1.5

80 1.0 60

0.5 Lch ≈ 1 µm

0.0 1.8

e

Wavelength (µm) 1.5 1.4 1.3 1.2 1.1 1

0.8

Tr(8,4)

Tr(8,3)

0.9

Ti/Pd-Ti/Pd

1.3

12 10

Ex(8,4)

0.9

Ex(8,3)

1.0 1.1 1.2 1.3 Photon energy (eV)

1.4

2.4 2.7 Vds (V)

3.0

40

3.3

2

14

Ti/Pd-Ti/Au

2.1

×10

16 (8,3) EL peak intensity (a.u.)

d

0.9 1.0 1.1 1.2 Photon energy (eV)

1.1

I(8,3) / I(8,4)

0.8

100 Ids (µA)

Luminescence intensity (a.u.)

b

EL integral intensity (a.u.)

a

Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 0.9 0.8

8

2

4 6 8 10 12 Device number

6 4 2 0

0

3 6 9 (8,4) EL peak intensity (a.u.)

2

12 ×10

Figure 1. (a) Cross-sectional representation of the CNT-film light emitter. From bottom to top, the structures are the Si wafer, Ag reflector, SiO2, HfO2, CNT active region, and PMMA layers. Source and drain electrodes are symmetric Ti (0.5 nm)/Pd (30 nm) metals. (b) EL spectra of trions from a typical free-space device. The EL peaks are from (8,3) CNT trion emission (1180 nm/1.05 eV) and (8,4) CNT trion emission (1340 nm/0.925 eV), respectively. The PL spectrum is also shown in the bottom panel, and the energy differences are labeled. (c) Dependence of integral EL intensity and Ids on Vds. The exponential fit (navy line) by impact excitation mechanism is also shown. (d) EL spectra from Ti/Pd-Ti/Pd contacts (top panel) and Ti/Pd-Ti/Au contacts (bottom panel). Tr represents the trion emission, and Ex represents the neutral exciton emission (gray lines). (e) Ratio of peak intensity from (8,3) CNTs I(8,3) versus that from (8,4) CNTs I(8,4). Inset: Statistic results from 12 devices show the stable EL characteristics.


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a

b

c

d

8

×10-4

6

6

10

data with Lch = 0.5 µm 1 µm 2 µm 4 µm 5 µm

5

10

4

10

0

5

10 15 Vds (V)

20

5

-4 ×10

6

4 ηEL

7

10

ηEL (photons per hole)

10 EL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

4 2

2

0

0

1000 2000 3000 Current density (Acm-2)

1

2

3 Lch (µm)

4

5

Figure 2. (a) Dependence of EL intensity and Vo on Lch. With increasing Lch from 0.5 μm to 7 μm, the Vo linearly increases from 1.2 V to 8 V (labeled by purple squares).

With increasing Lch (e.g., ~ 8 V), the EL intensity also obviously decreases by ~ 5 orders of magnitude (labeled by red dots). (b) Schematic of impact excitation. Top panel indicates that only a single excitation site is included in a short current flow path; Bottom panel indicates the tandem impact excitations in a longer current flow path. This comparison indicates the tandem impact excitation effect on EL. (c) Dependence of

integral emission intensity on Vds, with varied Lch from 0.5 μ m to 5 μ m. The experimental data and the corresponding fits based on the impact excitation mechanism

show the same trend. (d) Basically equal of ~ 4×10-4−6×10-4 (at the current density of ~ 1700−3000 Acm-2) as a function of Lch (Lch ≤ 5 μm). Inset: as a function of current density. The increased can be attributed to enhanced impact excitation rate

because of the increased Vds.


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18 V 13 V

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1592

1584 1580 1576 1572

9V

2

0V

1450

(eFthλph) /(kBT)

2

5V 3.5 V

1500 1550 1600-1 Raman shift of G mode (cm )

1650

800 700 600 500 400 300

1588

0

3

6

9

Temperature T (K)

b

-1

a

Spectral intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G-mode position (cm )

ACS Photonics

12 15 18

120 80 40 0

3

6

9 12 15 Vds (V)

18

Figure 3. (a) G-mode of Raman spectra at different Vds from a 1-μm-channel device. With increasing Vds from 0 V to 18 V, the mode shift can be observed from 1590 cm-1 to 1573 cm-1. (b) Top panel: Specific data of Raman peak (left) and estimated T (right). T at ~ 570 K can also be calibrated by a PMMA coating layer with a Td (~ 570 K, labeled by two violet arrows), which corresponds to the Vds of ~ 10−12 V. With increasing Vds from 0 V to 18 V, T will increase from ~ 300 K to ~ 800 K. At a larger Vds (~ 18 V/800 K), the device is close to oxidative damage induced by current heating.

Bottom panel: Ratio of ( '! /(kBT'! , which confirms local electric-field-induced EL mechanism.


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a

c

b

1330

1.0 0.5 0.0 0.8 0.4 0.0 1120 1200 1280 1360 1440 1520 Wavelength (nm)

d

1320 1310

0.0 1200

1300

1300 1400 Wavelength (nm)

1290 λc = 1285 nm

1280

e

1.0 Au thickness 15 nm 20 nm 30 nm 0.5

Normalized EL (a.u.)

Resonance wavelength (nm)

Normalized EL spectra (a.u.)

1.5

15

20 25 30 Top Au thickness (nm)

f 4

1260 1250

1.0

0.5

0.0

0

Angle θ 0° 10° 20° 30°

1050 1200 1350 1500 Wavelength (nm)

1200 1300 1400 Wavelength (nm)

5 10 15 20 25 Viewing angle θ (°)

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10

0

30

1100 1200 1300 1400 1500 Wavelength (nm)

30

λc = 1180 nm

27

10

2

Vds (V)

FWHM (nm)

1270

EL spectral profile (a.u.)

1280

33 EL peak intensity (a.u.)

λc = 1285 nm

EL spectral profile (a.u.)

10 Normalized EL spectra (a.u.)

1290 Output wavelength (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

3

4

Figure 4. (a) Top panel: Intrinsic EL spectrum from a free-space CNT emitter (labeled by the gray line); Bottom panel: Measured spectra from four cavity-integrated devices with the D = 1180 nm, 1285 nm, 1339 nm and 1443 nm, respectively. (b) Finite-element

simulation of the E-field distribution within the λ/2 cavity. From bottom to top, black-line frames outline the Ag reflector, SiO2, HfO2, PMMA, Au reflector, and free-space air. (c) Dependence of the on-axis resonance wavelength on the thickness of top Au reflector. Inset: Spectral blueshift from 1320 nm to 1285 nm due to different Au thicknesses. (d) Output wavelength as a function of the viewing angle J. Inset: Specific spectra from 1285 nm to 1256 nm under different J. (e) Typical on-axis EL spectra from a cavity-integrated

device with Lch = 0.5 μm and D = 1180 nm, at different Vds from 1.5 V to 4 V. Inset: Optimized spectral profile due to cavity effect (red line), compared with results from the trion emission (dash fit line) of (8,3) CNTs and free-space emission (gray line). (f) Exponential dependence of the peak intensity on Vds (shown in logarithm) in a 0.5-μmchannel device, and the constant FWHM (~ 30 nm/28 meV).


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