Diethylenetriamine-Doped Graphene Oxide Quantum Dots with

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Cite This: ACS Appl. Nano Mater. 2019, 2, 3925−3933

Diethylenetriamine-Doped Graphene Oxide Quantum Dots with Tunable Photoluminescence for Optoelectronic Applications Svette Reina Merden S. Santiago,† Chiao-Hsin Chang,‡ Tzu-Neng Lin,† Chi-Tsu Yuan,† and Ji-Lin Shen*,† †

Department of Physics and Center for Nanotechnology and ‡Master Program in Nanotechnology at CYCU, Chung Yuan Christian University, Chung-Li 32023, Taiwan

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S Supporting Information *

ABSTRACT: Modulation of the photoluminescence (PL) properties in graphene oxide quantum dots (GOQDs) is important for fundamental research and applications in bioimaging and optoelectronic devices. Herein, the synthesis of GOQDs doped with diethylenetriamine (DETA) was implemented by one-pot synthesis via oil bath heating. The PL intensity in the GOQDs was increased by an enhancement factor of ∼13.2 following the DETA doping. The band gap of the GOQDs increased with the increase in DETA doping, producing tunable PL from red to blue emission with a wavelength change of ∼265 nm. The significant tuning of the PL wavelength in the GOQDs was attributed to the variation of pyrrolic and/or graphitic N in the C−N bonding configuration. The synthesized GOQDs were found to inject carriers into the gallium nitride (GaN) epilayer in the GOQD/GaN composite, which in turn enhance the PL in the GaN epilayer. Our observations provide effective approaches for tuning the PL wavelength in GOQDs and enhancing the PL intensity in GaN; these are promising developments for applications in optoelectronic devices such as light-emitting diodes and solar cells. KEYWORDS: graphene oxide quantum dots, doping, photoluminescence, oil bath, diethylenetriamine, GaN luminescence.3 Moreover, GOQDs may be dissolved more readily in a hydrophilic or hydrophobic solution due to the carboxyl and carbonyl groups.3 GOQDs are thus a promising candidate for utilization in many applications such as photocatalysts in water splitting and as photosensitizers in photovoltaics.16 Tunable photoluminescence (PL) is important in the fabrication of graphene-based nanostructures since it involves applications in bioimaging, sensing, and optoelectronics. Many studies have been focused on the tunable PL in GQDs.5,8−10,17,18 Heteroatoms and molecular doping into GQDs may tailor the band gap of GQDs and affect their luminescence. For example, by introducing ammonia or PEGdiamine, we obtained the amine-doped GQDs through chemical cutting of GO in the top-down route.19,20 The GQDs modulated by amino groups can tailor the band gap of GQDs and tune their PL.19,20 However, to our knowledge, little attention has been given to the tunable PL in GOQDs. Doping into GOQDs is expected to tune the PL wavelength of GOQDs since the increase of dopant densities has been demonstrated to alter the band gap of GO and/or GQDs.18,21 Investigations of the tunable PL properties in GOQDs not only provide an essential understanding of their fundamental

1. INTRODUCTION Because of the popularity of graphene in potential applications, similar graphene-based materials have followed the trend in fundamental and industrial research. Graphene quantum dots (GQDs)a new graphene derivativehave been discovered and are catching the attention of various researchers due to their peculiar properties such as edge effects and quantum confinement.1−4 GQDs emit luminescence ranging from the blue to the red spectral region; the range can be controlled by their size, shape, oxygen-related functional groups, and redox potential.5−7 Recently, a high fluorescence quantum yield (QY) has already been achieved and reported in GQDs synthesized by pulsed laser ablation, the microwave sequential bottom-up route, or low-temperature pyrolysis.8−10 Graphene oxide (GO) is a graphene sheet modified with epoxy and hydroxyl groups on the basal plane and carbonyl and carboxyl groups at the edges. GO has been attracting significant academic research focus owing to its peculiar physical and/or chemical properties and promising practical applications.11−13 Graphene oxide quantum dots (GOQDs) are minute pieces of GO that emit steady-state blue or yellowish-green luminescence.4,14,15 It is challenging to distinguish GQDs and GOQDs since both of them contain sp2 carbon domains with oxygen-related functional groups; however, it has been reported that GOQDs reveal better photoluminescence (PL) properties than GQDs because of the reduction in intrinsic luminescence and enhancement in defect © 2019 American Chemical Society

Received: April 29, 2019 Accepted: May 30, 2019 Published: May 30, 2019 3925

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

Article

ACS Applied Nano Materials properties but also make them particularly useful for applications in light-emitting diodes as the luminescent component and in solar cells as the photosensitizer. Herein, we introduce a facile one-pot approach for synthesizing GOQDs with tunable band gap by doping diethylenetriamine (DETA). A change of PL wavelength as large as ∼265 nm has been implemented in the DETA-doped GOQDs by varying the DETA concentration. Characterization techniques such as transmission electron microscopy (TEM), atomic force microscopy (AFM), PL and time-resolved PL (TRPL), ultraviolet−visible (UV−vis) absorption, Fourier transform infrared (FTIR) absorption, Raman scattering, Xray photoelectron spectroscopy (XPS), and gate voltagedependent conductance measurement were employed to investigate the properties of the synthesized GOQDs. The mechanism involving the shift of PL wavelength is discussed below. Recently, the luminescence of GaN-based semiconductors has been extensively studied due to its applications in lightemitting diodes and lasers. To further improve the GaN-based devices, enhancement of luminescence in GaN is desirable. It is known that charge carriers can transmit from GQDs to semiconductors due to a work-function difference via the deposition of GQDs on GaN.22 The effects of the DETAdoped GOQDs on the PL of GaN epilayers were studied herein. The PL intensity of GaN increased as the DETA-doped GOQDs were injected on top of the GaN epilayer. Based on the time-resolved PL (TRPL) measurements, the mechanism that leads to the increase of PL in GaN is also addressed.

Figure 1. (a) Schematic diagram of the DETA-doped GOQD synthesis. (b) Images of the DETA-doped GOQD solution under UV lamp with increasing DETA doping concentration (left to right). scattering and FTIR, respectively. UV−vis absorption spectra were measured by using the JASCO V-750 spectrometer. The steady-state PL properties were investigated by using a FluoroMax-4 (Horiba Jobin Yvon) spectrometer. The fluorescence QY was measured with a FluoTime 300 (PicoQuant) spectrometer utilizing the 375 nm excitation wavelength. A pulsed laser with a 150 fs duration, a 260 nm wavelength, and a 20 MHz frequency was utilized for studying TRPL. In the TRPL measurement, the PL was analyzed by a 0.75 m spectrometer and focused to a high-speed photomultiplier tube with an instrument response around 200 ps. To measure the gate voltagedependent conductance, electrodes were patterned by optical lithography on a p+ Si substrate with a SiO2 layer of 100 nm thickness, followed by the evaporated Ti/Al films.

2. EXPERIMENTAL SECTION In this experiment, GO solution (500 mg/L concentration) purchased from Graphene Supermarket and DETA (0.955 g/mL concentration) purchased from Sigma-Aldrich were used as the GOQDs and doping sources in a one-pot synthesis method, respectively. A 500 μL aliquot of aqueous GO solution was put in a 50 mL single-neck, flat-bottom round flask with 7 mL of deionized (DI) water. The solution was added with different DETA concentrations (25−371 mM), mixed through a vortex shaker at 6000 rpm, and then placed under an oil bath at a temperature of 170 °C for 30 min. A 4 mL hydrogen peroxide (H2O2) solution, purchased from Honeywell, was added gradually during the heating process. The product was then cooled to room temperature and was washed by DI water and dialyzed (0.1−0.5 kDa membrane) for 48 h to remove untreated GO sheets and large particles. The stock solution of GOQDs obtained had a concentration of ∼1.7 mg/mL. Pristine GOQDs were prepared with the same procedure without the addition of DETA. A schematic of the preparation process is shown in Figure 1a. The n-type GaN epitaxial layer was grown by metal−organic chemical vapor deposition (MOCVD) where ammonia (NH3), trimethylgallium (TMGa), and silane (SiH4) were utilized as N, Ga, and Si sources, respectively.22 The epitaxial layers consisted of a 30 nm GaN nucleation layer on a sapphire substrate, a 2 μm GaN buffer layer, and a 1 μm Si-doped n-GaN layer with the doping concentration of 5 × 1018 cm−3. GOQDs with a volume of 2.5 μL were drop-cast onto the surface of GaN using a pipet and dried on a laboratory grade oven at 60 °C for 20 min. Subsequent doping was performed by repeating the same method for all DETA-doped GOQDs. For characterizations of the GOQD properties, a JEOL JEM-2100F electron microscope with an operating voltage of 200 kV, a psia XE100 system, and a Thermo Scientific K-Alpha ESCA instrument equipped with a monochromatic Al Kα X-ray source at 1487 eV were used to examine the TEM, AFM, and XPS, respectively. A Horiba Jobin Yvon (iHR320) spectrometer equipped with a 532 nm laser and a Jasco FTIR-4100 spectrometer were used to analyze the Raman

3. RESULTS AND DISCUSSION During the synthesis with oil bath heating, GO was observed to disintegrate after a few minutes of treatment, caused by the thermal decomposition of the reaction. Because of the introduction of DETA in the precursor, smaller GO sheets interact with them and become DETA-doped GOQDs. The production yield of GOQDs, defined as the weight of GOQDs after evaporating solvent relative to the weight of the GO sheets (containing nanoflakes, nanoparticles, and GOQDs), is around 43%. The as-synthesized GOQDs with increasing the DETA concentration from 0 to 371 mM were photographed under UV light, displaying fluorescence from faint dark-red to bright blue emission (Figure 1b). Typical TEM, HRTEM, and the size distribution of the pristine GOQDs are shown in Figures 2a−c, respectively. The pristine GOQDs were observed to be monodispersed as seen in Figure 2a. A fine crystalline structure with a spacing of ∼0.23 nm was found in HRTEM (Figure 2b). An average diameter of 3.3 ± 0.3 nm for GOQDs was obtained (Figure 2c). Similarly, TEM, HRTEM, and the size distribution for the DETA-doped GOQDs with the DETA concentration of 371 mM are shown in Figures 2d−f, respectively. The TEM images of the GOQDs doped with the DETA concentration of 25, 127, and 247 mM are also shown in Figure S1. The HRTEM image of the DETA-doped (371 mM) GOQDs reveals a fine crystalline structure with a lattice spacing of ∼0.22 nm, as marked by the arrow in Figure 2e. The average diameter of the DETA-doped GOQDs was found to be 2.7 ± 0.2 nm, as 3926

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

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Figure 2. (a) TEM image, (b) HRTEM image, and (c) the average diameter of the pristine GOQDs. (d) TEM image, (e) HRTEM image, and (f) average diameter of the DETA-doped GOQDs (371 mM).

Figure 3. AFM images of (a) GO, (b) the pristine GOQDs, and (c) the DETA-doped GOQDs (371 mM). Inset of (b) and (c): height profiles corresponding to the lines in (b) and (c), respectively.

Figure 4. (a) XPS survey spectra of the DETA-doped GOQDs with varying concentration of DETA. (b) N/C atomic ratio of the GOQDs vs the DETA concentration.

shown in Figure 2f. The above TEM and HRTEM data reveal that the size of GOQDs synthesized from our one-pot method via oil bath heating is in good agreement with those GQDs synthesized from other techniques.2,4,9,10,12 Figures 3a−c display the AFM pictures of GO, the pristine GOQDs, and the DETA-doped GOQDs, respectively. The insets of Figures 3b and 3c show the height profiles of several QDs chosen from the pristine and DETA-doped GOQDs, respectively. The

heights of the GOQDs were around 1−2 nm, corresponding to a few layers thick. Figure 4a shows the XPS survey spectra of the GOQDs with increasing DETA doping concentration. The XPS spectra of pristine GOQDs display evident peaks located at ∼286.5 eV (C 1s) and ∼533.1 eV (O 1s), representing oxygen and carbon which are found similarly in other GQDs and GOQDs reported previously.23,24 Following the introduction of DETA, two peaks associated with N were observed at ∼400.5 and 3927

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

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Figure 5. XPS N 1s spectrum of the DETA-doped GOQDs with the doping concentrations of (a) 25, (b) 185, and (c) 371 mM. (d) Dependence of the N-bonding configurations of DETA-doped GOQDs on DETA concentrations.

∼406.9 eV, which will be discussed in the next paragraph. The N/C atomic ratio of the GOQDs versus the doping concentration is shown in Figure 4b. The highest N/C atomic ratio is 5.3% for the GOQDs with the DETA concentration of 371 mM. The high-resolution N 1s spectra of DETA-doped GOQDs with the DETA concentration of 25, 185, and 371 mM are shown in Figures 5a−c, respectively. The broad N peak was deconvoluted into three subpeaks related to different carbon to nitrogen bond configurations: pyridinic N (∼399.1 eV), pyrrolic N (∼400.5 eV), and graphitic N (∼402.1 eV). Herein, the pyridinic N, the pyrrolic N, and the graphitic N were assigned to sp2-hybridized bonding with two C atoms, sp3hybridized in the five-atom ring, and direct N substitution replacing a C atom within the graphite lattice, respectively.25−27 By extracting from Figure 5a−c, we found that pyrrolic N is dominant as compared to the pyridinic and graphitic N. The peak at 406.9 eV was assigned to the binding energy of the NO3− (N5+) species, which is likely related to the oxidation of the N species.28 An analysis of different N configurations with increasing doping concentrations in DETA-doped GOQDs is shown in Figure 5d. The pyrrolic N (graphitic N) increases (decreases) with increasing the DETA concentration. On the other hand, the pyridinic N changes minutely with varying the DETA concentration. It is noted that the main N signal in the XPS spectra (∼400.5 eV) could be attributed to the amine N in DETA.29 In this case, the N in DETA is the noninteracting amine N instead of the N grafted on the GOQD. To verify whether or not the N in DETA is the noninteracting N, the FTIR spectra were investigated. Figure 6 shows the FTIR dips of the pristine GOQDs, the DETA-modified GOQDs, and the pure GO. Various dips at 1280, 1385, 1580, 1850, and 3450 cm−1 were detected in the FTIR of GO and assigned to the C−O−C

Figure 6. FTIR spectra of GO (green), pristine GOQDs (red), and DETA-doped (371 mM) GOQDs (blue).

(epoxy), OH (bending vibration), CC (aromatic ring), C O (nonconjugated ketone), and OH (stretching vibration), respectively.30−32 For the pristine GOQDs, the absorption bands are similar to those of GO, except for the disappearance of the OH bending vibration. After incorporation of DETA, two new absorption bands occurred at ∼1015 and ∼1650 cm−1, attributed to the C−N in-plane and HNCO (amide I) stretching vibration, respectively.32,34,35 The appearance of these two bands indicates that the amine of DETA had been successfully grafted upon the GOQDs. Another method used to examine whether the amine of DETA does interact with GOQDs is to measure the doping effect in GOQDs; this will be proved later by the gate voltage-dependent conductance measurements. On the basis of the FTIR result, we suggest that the reaction mechanism between GO and DETA may involve the cutting and passivation processes.36 In the synthesis processes, the GO nanosheets were cut by H2O2 into smaller pieces since H2O2 is a strong oxidizing agent.37,38 The carboxylic acid groups in the 3928

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

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Figure 7. (a) Optical absorption spectra of the DETA-doped GOQDs with varying concentrations of DETA. (b) Curves of (αhν)2 vs photon energy (hν) of the pristine (black) and DETA-doped GOQDs (371 mM) (green). (c) Band gap energy of the DETA-doped GOQDs with increasing DETA concentrations.

small GO fragments then react with the amine groups of DETA through the amidation process, as evidenced by the appearance of the 1650 cm−1 peak in the FTIR of the DETAdoped GOQDs (Figure 6). The amidation reaction passivates the active sites of GO surfaces, suppressing the surface defects originated from the oxidation cutting of the GO nanosheets. Thus, both H2O2 and DETA play important roles in producing the DETA-doped GOQDs. Figure S2 displays the Raman spectra of GOQDs with varying DETA concentrations. Two peaks at ∼1350 and ∼1590 cm−1 were observed in pristine GOQDs, corresponding to the D band and the G bands, respectively.39 After the introduction of DETA, a peak at ∼1625 cm−1 appeared and merged with the G peak for higher DETA concentration. This new peak has been observed in the amino-acid-modified GO and is attributed to the N doping in GO.40 Thus, the appearance of the 1625 cm−1 peak provides further evidence for the successful N doping in GOQDs. The UV−vis spectroscopy of the pristine GOQDs is shown in Figure 7a, displaying a peak at ∼230 nm and a tail extending to the red spectral region. The peak found at ∼230 nm was attributed to the π−π* transition of CC bonds of aromatic sp2 domains within the GOQDs.37 As the DETA concentration increases, the absorption tail was found to shift toward the short-wavelength region. The energy band gap Eg of GOQDs can be estimated from the optical absorption tail via the Tauc expression:41 αhν = A(hν − Eg )1/2

Figure 8. (a) PL spectra of the DETA-doped GOQDs with varying concentration of DETA. The arrow reveals the direction for increasing DETA concentration. (b) QY of the DETA-doped GOQDs as a function of the DETA concentration.

enhanced from 0 to 371 mM. The maximum QY of the DETA-doped GOQDs attains 14%, indicating an enhancement factor of ∼13.2 as compared to the QY of the pristine GOQDs. Photostability of the pristine and DETA-doped GOQDs has been examined under illumination of a CW laser with a 375 nm wavelength. After 120 min of exposure of laser light, little decrease in the PL intensity (∼3%) was observed for the pristine and DETA-doped GOQDs, as shown in Figure S3, indicating that both GOQDs reveal good photostability under continuous UV light excitation. In general, the PL enhancement of GOQDs by introducing DETA may originate from two factors: doping or capping effects. For the doping effect, DETA provides additional carriers into GOQDs for carrier recombination and results in increased PL intensity. For the latter (capping) effect, DETA induces surface passivation of the reactive sites, which removes nonradiative recombination centers and increases the PL efficiency. To determine whether DETA serves as a dopant, the gate-voltage-dependent conductance was measured. Figure 9

(1)

where α is the weak-field absorption coefficient, h is Planck’s constant, ν is the frequency, and A is a constant. By plotting (αhν)2 versus hν from the absorption spectrum, we can obtain Eg from the x-axis intercept of the absorption edge, as shown in Figure 7b. By use of such an approach, Eg as a function of the DETA concentration was obtained and is shown in Figure 7c. Eg increases monotonously with increasing DETA concentrations. Figure 8a shows the PL spectra of GOQDs with varying DETA concentrations. The PL shifts to the high-energy side when the DETA concentration increases, which agrees well with the results in the optical absorption spectra (Figure 7). The step-by-step change of PL reveals that the tunable PL can be implemented by adjusting the amount of dopant concentration. Developing the bright GOQDs with tunable PL is important because changes in the PL wavelength are advantageous for practical applications. Figure 8b shows the QY of GOQDs as a function of the DETA concentration. The QY was enhanced when the DETA concentration was

Figure 9. ID−VG curves of the pristine (blue) and DETA-doped (red) GOQDs. 3929

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

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Figure 10. (a) PL spectra of pure GaN epilayer (black) and the DETA-doped-GOQD/GaN composite with the DETA concentration of 0, 25, 185, and 371 mM. (b) PL intensity (PL decay transient) ratio of the GaN with the DETA-doped GOQDs to that of the untreated GaN with varying concentration of DETA. (c) PL decay transients of pure GaN epilayer (black) and the DETA-doped-GOQD/GaN composite with the DETA concentration of 0, 185, and 371 mM. Red lines in (c) represent the fits using eq 2.

shows the drain current Id versus the gate voltage Vg for the GOQDs without and with the introduction of DETA. The Id for the DETA-doped GOQDs enhances ∼100-fold when compared to the pristine one. The on−off current modulation ratio (i.e., Ion/Ioff, where Ion and Ioff represents the on-state and off-state drain current, respectively) of GOQDs increases after the introduction of DETA. Also, the threshold voltage of the Id−Vg curve changes toward the positive Vg when DETA is incorporated in GOQDs. The increase in Ion/Ioff and the positive change in the threshold voltage indicate the successful p-type doping of GOQDs with DETA. The p-type doping for the GOQDs with DETA could be attributed to the existence of oxygen-related functional groups or physisorbed oxygen adsorbates in air.42,43 On the other hand, the amidation reaction can passivate the active sites of GO surfaces and suppress the surface defects.38 Therefore, both the doping or capping effects could lead to the PL enhancement in GOQDs after introducing DETA. Figure S4a displays the PL of the pristine GOQDs measured at the excitation wavelengths from 400 to 500 nm. The peak wavelength of PL in the pristine GOQDs remains at ∼690 nm with the variation of the excitation wavelength, which indicates excitation-wavelength-independent behavior. For the DETAdoped GOQDs (371 mM), the peak wavelength of PL changes from 430 to 570 nm with increased excitation wavelength from 340 to 480 nm, indicating dependence of the excitation wavelength on the PL wavelength (Figure S4b). To understand the dependence of the excitation wavelength on PL, the PL decay transients were investigated under the detection of various PL energies. The PL decay transients were fitted by using a stretched exponential function: I(t ) = I(0)e−(kt )

pristine GOQDs is independent of the emitting energy; this reveals that the localization of carriers is insignificant for the pristine GOQDs. We suggest that the doping in the synthesis process may lead to some localized states, which produce carrier localization in the recombination process. Thus, the DETA-doped GOQDs reveal the dependence of the excitation wavelength on the PL. The tunable PL of GQDs induced by N doping has been studied recently.18,25,42 The N doping of GQDs with O-rich functional groups causes the blue-shift of PL and possesses superior electrocatalytic ability.44,45 Also, the PL of GQDs is highly influenced by the n−π* transition, originated from the N-containing aromatic rings and conjugate structure of graphene.46 For the amino-acid-doped GQDs, the tunable PL emission between green and blue regions was observed, explained by the existence of an additional interband within the energy gap induced by the orbital hybridization of C−N atoms.47 In our case, the PL emission of GOQDs was shifted from the red to blue light with increased DETA concentration. The wavelength shift of PL emission due to N doping (∼265 nm) is larger than those reported previously (23−194 nm).18,20,43 The blue-shift of PL due to the N doping in GQDs has been attributed to the quantum confinement effect, the O-rich groups, and the strong electron-withdrawing ability of the N atoms.18,20,43 From our TEM data (Figure 2), the average diameter of the DETA-doped GOQDs (∼2.7 nm) is smaller than that of the pristine GOQDs (∼3.3 nm). According to the quantum confinement calculation, the reduction in diameter from about 3.3 to 2.7 nm would lead to an enhancement of the band gap of GQQDs by ∼0.15 eV.48 This energy variation is much smaller than the energy change of PL in our GOQDs after DETA doping (∼1.2 eV); thus, the quantum confinement effect is not the major origin for the wavelength shift of PL in GOQDs. On the other hand, from the XPS measurements, the O/C atomic ratio of GOQDs changes little (∼8%) after the DETA doping (Figure 4a), indicating that the O-rich groups are not the major cause for the blue-shift of PL in GOQDs since the O-rich groups remain unchanged after DETA doping. It has been reported that the high electronegativity of graphitic N can produce a strong negative induction effect, reducing the π electron cloud and broadening the band gap of GQDs.17 However, this cannot explain the blue-shift of PL in our GOQDs since the content of graphitic N decreases as the DETA doping increases (Figure 5d). Thus, the above three effects are not responsible for the blue-shift of PL in the DETA-doped GOQDs. Recently, the consequences of different N-doping configurations on the band gap of GQDs have been calculated on the basis of density-

β

(2)

where I(0), k, and β are the initial PL intensity, the decay rate of PL, and a dispersive factor, respectively. The PL decay time τ was obtained by the equation8

τ=

1 ijj 1 yzz Γj z kβ jk β z{

(3)

where Γ is the Gamma function. The PL decay time versus the emitting energy for the pristine and DETA-doped GOQDs is plotted in Figures S4c and S4d, respectively. With increased emitting energy, the PL decay time in pristine GOQDs remains unchanged (∼0.48 ns), while that of DETA-doped GOQDs changes from 3.8 to 2.6 ns. The decrease in PL decay time as the emitting energy increases reveals a feature of carrier localization.8 On the other hand, the PL decay time in the 3930

DOI: 10.1021/acsanm.9b00811 ACS Appl. Nano Mater. 2019, 2, 3925−3933

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ACS Applied Nano Materials functional theory.5 Pyridinic N-doping on GQDs is found to produce a slight blue-shift of emission from 572.4 to 550.3 nm with the N-doping of 12.5%. In addition, the pyrrolic N atoms situated at the edges of GQDs substantially broaden the band gap. A 9.09% of pyrrolic N-doping induces the wavelength change of emission in GQDs from 624.4 to 575.5 nm. On the other hand, graphitic N-doping at the edge or center significantly decreases the band gap of GQDs and quenches their luminescence. According to the above argument and Figure 5d, we suggest that the increase of pyrrolic N and/or the decrease of graphitic N lead to the broadening of the band gap in GOQDs. The red to blue light from the PL of DETAdoped GOQDs can thus be explained by the variation of pyrrolic and/or graphitic N after doping. Figure 10a shows the PL spectra of the pure GaN epilayer and the DETA-doped-GOQD/GaN composite with different DETA concentrations. Under the excitation light with the wavelength of 260 nm, the PL intensity of GaN is enhanced following the deposition of GOQDs on top of the GaN epilayers. The enhancement ratio of PL (I/I0, where I and I0 are the PL of GaN in the presence and absence of GOQDs, respectively) increases with increased DETA concentrations, as shown in Figure 10b (open circles). The maximum PL of GaN occurs after the deposition of DETA-doped GOQDs with a concentration of 371 mM, attaining an enhancement of ∼6. The increase of PL intensity in the GaN could prove it valuable for practical applications in optoelectronic devices. To discover the mechanism of PL intensity enhancement in GaN, we studied the time-resolved PL. Figure 10c shows the PL transients of the pure GaN epilayers and the dopedGOQD/GaN composite with different DETA concentrations. The PL decay curves were analyzed by eq 2, and the results are displayed as the solid lines in Figure 10c. The PL decay lifetimes in GaN can be calculated according to eq 3; the results are displayed in Table 1. The PL decay time of GaN

enhancement of GaN by the deposition of GQDs has been observed and explained by the transfer of carriers from GQDs to GaN.22

4. CONCLUSION The GOQDs doped by DETA with an average particle size of around 2.7−3.3 nm were successfully synthesized by the onepot synthesis with oil bath heating. The PL intensity of the GOQDs was enhanced with a factor of ∼13.2 after doping of DETA with the concentration of 371 mM. The DETA-doped GOQDs reveal a tunable PL from red to blue emission with a change of ∼265 nm in the PL wavelength. The tunable PL from the DETA-doped GOQDs is related to the variation of pyrrolic and/or graphitic N after doping. In addition, GOQDs can be used as carrier injectors, which inject carriers into the GaN epilayer and enhance the PL of GaN in the GOQD/GaN composite. This study provides effective approaches for the wavelength tuning of the PL in GOQDs and the enhancement of PL in GaN, encouraging results for practical applications in the optoelectronic devices such as light-emitting diodes and solar cells.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00811.

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TEM images and Raman spectra of the GOQDs with varying DETA doping concentrations; and photostability, PL spectra and the dependence of the emitting energy on the PL decay time for pristine and DETAdoped (371 mM) GOQDs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Table 1. Dispersion Component (β), the Decay Rates (k), and the Decay Lifetime (τ) of the GaN after Introduction of the GOQD bare GaN pristine GOQDs/GaN DETA-doped GOQDs(185 mM)/GaN DETA-doped GOQDs(371 mM)/GaN

β

k (ns−1)

τ (ns)

0.7 0.68 0.55 0.53

7.692 6.897 6.667 6.061

0.165 0.189 0.255 0.298

ORCID

Chi-Tsu Yuan: 0000-0003-3790-9376 Ji-Lin Shen: 0000-0003-2881-712X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported in part by the Ministry of Science and Technology (Taiwan) under Grant MOST 106-2112-M033-009-MY3.

rises with the increased DETA concentration in the dopedGOQD/GaN composite. The open squares in Figure 10b show the enhancement ratio of the PL decay time (τ/τ0, where τ and τ0 are the PL decay time in the presence and absence of GOQDs, respectively). It was found that by increasing the DETA concentration, the enhancement ratio of the PL decay time (open squares in Figure 10b) is associated with the enhancement in PL intensity (open circles in Figure 10b). We infer that the increased PL intensity in GaN epilayers originated from the increased carriers injected from GOQDs since an increase in the PL decay time is related to an increase in the carrier density. The PL intensity and PL decay time were thus enhanced since the carriers in the GaN epilayers were increased. Because more and more carriers can be injected from GOQDs into GaN due to higher DETA doping, the enhancement ratios of PL intensity and PL decay time increased, as displayed in Figure 10b. A similar PL intensity

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