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Yang et al. Microstructures 2023;3:2023005 https://dx.doi.org/10.20517/microstructures.2022.24 Page 3 of 10

RESULTS AND DISCUSSION
Before investigating the spectral response characteristics of the graphene/ZnO QD photodetector, we first
study the photoresponsive behavior of the bare graphene device. A schematic diagram of the bare graphene
photodetector is displayed in Figure 1A. In this structure, graphene functions as the charge transport
channel as well as the light-absorbing medium. The Raman spectrum of graphene is shown in Figure 1B.
-1
The two predominant peaks located at 1583 and 2680 cm can be attributed to the graphene G- and
-1
2D-bands, respectively. The small peak at 1344 cm is the D-band Raman peak of graphene, related to
defects or disorders in the hybrid structure [49,50] . The intensity ratio between the 2D and G peaks is ~0.6,
indicating that the graphene is bilayer . Figure 1C demonstrates the photoresponsive behavior of the bare
[51]
graphene device under UV and visible illumination at a source-drain voltage (V ) of 0.2 V and a gate
DS
voltage (V ) of 0 V. Under illumination at all wavelengths, the source-drain current (I ) gradually decreases
DS
G
within 1000 s, indicating persistent NPC. The persistent NPC in graphene derives from the adsorbents (O
2
or OH groups) at its surface, which induce p-type conductance and act as scattering centers under
-
illumination, thereby reducing its conductance [24,52] . The decrease in free carrier mobility of graphene under
illumination can also be confirmed by Hall measurements. In dark conditions, the hole mobility of
graphene is 1050 cm V s , which decreases to 910 cm V s under 365 nm UV illumination. After the
2
-1
-1
2
-1
-1
light is switched off, the I recovers slowly. For instance, after 4000 s, the current under 365 nm
DS
illumination only recovers to 24% . It is notable that both the saturated current and transition time vary
[53]
with the incident light wavelength, which is related to the wavelength-dependent absorption characteristics
of graphene [24,54] . Figure 1D shows the responsivity of the graphene photodetector vs. the excitation
wavelength. The responsivity is defined as R = (I -I )/(P *S), where I is I under illumination, I is I
ph
DS
ph
ph dark
DS
dark
in the dark environment, P is the incident light intensity density and S is the active area of the device. The
ph
negative responsivity values represent the NPC behavior of the photodetector. The absolute value of the
responsivity increases as the excitation wavelength decreases.
To make the response of the photodetector to UV illumination distinguishable from that to visible
illumination, the graphene is coated with ZnO QDs to form a graphene/ZnO QD photodetector, as shown
in Figure 2A. Figure 2B presents the transmission electron microscopy (TEM) image of the ZnO QDs,
which show a typical diameter of ~5 nm. The HRTEM image clearly identifies lattice fringes with a spacing
of 0.26 nm, corresponding to the (002) plane of hexagonal wurtzite ZnO . Figure 2C shows the XRD
[55]
pattern of the ZnO QDs. There are six characteristic peaks, corresponding to the (100), (101), (102), (110),
(103) and (200) planes of the hexagonal wurtzite structure of the ZnO QDs (P63mc, a = b = 0.325 nm, c =
0.521 nm, JCPDS Card No. 80-007). Figure 2D-F show the XPS survey, O 1s and Zn 2p spectra of the ZnO
QDs. The O 1s spectrum is composed of three peaks at 530.8, 532.0 and 534.0 eV, corresponding to Zn-O,
O-C=O and C-O-C/OH bonds, respectively . The XPS spectrum for Zn 2p shows two peaks at 1023.4 and
[56]
1046.5 eV, which can be attributed to Zn 2p and Zn 2p , respectively, thereby confirming the divalent
3/2
1/2
state of Zn. Figure 2G shows the Raman spectrum of the graphene/ZnO QDs. In addition to the peaks from
-1
graphene, there are three obvious peaks at 99, 418 and 640 cm , which derive from the E2(low), E1(TO) and
multi-phonon modes of ZnO, respectively [57,58] . Figure 2H presents the absorption of the graphene/ZnO
QDs, which show an absorption edge at ~376 nm, corresponding to the bandgap of ZnO. The absorption in
the visible region is due to scattering by the ZnO QDs.
Figure 3A shows the response characteristics of the graphene/ZnO QD photodetector under UV (upper
panel) and visible (lower panel) illumination. NPC behavior can be observed under visible illumination,
similar to that of the bare graphene photodetector. However, under UV illumination, the I increases
DS
gradually, indicating PPC response to UV illumination. After the light is switched off, the I recovers to its
DS
original level slowly. The current under 650 nm illumination recovers to 21% after 1400 s, while the current