Germanane and (3‐Hydroxypropyl)germanane as Single Crystal Photodetectors

2D materials are being widely investigated for their potential application in light‐detecting devices. Very recently, a spin‐coated flexible 2D germanium‐based photodetector with great photocurrent density and responsivity as well as short rise and decay time is presented by Liu et. al. In this paper, the solid‐state single flake photodetector base on either pure germanane or (3‐hydroxypropyl)germanane is reported. Both prepared photodetectors while irradiated with different wavelengths of light exhibit outstanding responsivity of 1157 mA W−1 for GeH and 362 mA W−1 for (3‐hydroxypropyl)germanane, which exceeds Liu's flexible reported GeH photodetector (22 µA W−1) by more than five orders of magnitude. These constructed photodetectors also exhibit rise and fall times lower than 20 ms, which is also significantly faster than the GeH photodetector reported by Liu with rise and decay times 240 and 740 ms, respectively.


Introduction
The last two decades have experienced enormous hunt for new 2D materials which revealed their unique properties such as high conductivity (thermal and electronic), [1] ballistic transport, [2] Hall effect, [3] etc. Starting with the rediscovery of graphene in 2004 by Geim and Novoselov, [3] the family of 2D materials as graphene, hexagonal boron nitride (h-BN), [4] semiconductors from the Group 15, [5] transition-metal dichalcogenides (MoS 2 , WS 2 , etc.), [6] transition-metal oxides, [7] thiophosphides, [8] metal chalcogenides [9] and young family of MXenes [10] has been growing ever since. Very recently, monoelemental 2D materials have been investigated. 2D pnictogens [11] such as black phosphorus, [12] arsenene, [13] antimonene [14] and bismuthine have been extensively studied. The Group 14 materials based on DOI: 10.1002/adom.202300288 silicon and germanium, i.e., silicene and germanene represent a very promising class of materials but, unfortunately, only with limited stability and applicability. Germanene fully hydrogenated analogue, i.e., germanane was first synthesized by Goldberger in 2013. [15] Since then, several procedures for germanane functionalization have been reported including direct exfoliation of CaGe 2 with alkyl halides as well as Ge-H bond activation using sodium naphthalenide followed by its subsequent alkylation. [16] Functionalized germananes exhibit a bandgap between 1.59 eV (for Ge-H) and 2.0 eV (for alkyl modified germananes), [16a,17] thus allowing their possible application as active layers in photodetectors. In 2021, Ng et al. presented high photocurrent densities for several functionalized germananes as well as for Ge-H. [18] Liu et al. reported first use of Ge-H flexible photodetector prepared by spin coating of Ge-H suspension on polymeric support. Such photodetector exhibited outstanding photocurrent density of 2.9 μA cm −2 , responsivity of 22 μA W −1 as well as short rise time 0.24 s and decay time 0.74 s. [19]

Results and Discussion
In this paper, single flake solid state simple two-terminal photodetectors were fabricated from Ge-H and (3hydroxypropyl)germanane (Ge-(CH 2 ) 3 OH), and their optoelectronic properties were investigated. The device structure and its biasing conditions are schematically shown in Figure 1A. The optical image of the fabricated Ge-H photodetector is shown in Figure 1D. The as-received Ge-H flake is first mechanically (Scotch tape) exfoliated to create a fresh and flat surface. We note that it has not been possible to transfer ultrathin Ge-H flakes from the tape directly onto a clean substrate, such as SiO 2 /Si, HfO 2 /Si or Al 2 O 3 . We observed a lack of adhesive forces between the Ge-H surface and the tested substrates (SiO 2 /Si, HfO 2 /Si of Al 2 O 3 ). Ongoing research is currently focused on appropriate Ge-H surface functionalization that would facilitate the adhesion to a substrate. In this communication, we used the following strategy: the exfoliated flakes were attached to SiO 2 /Si substrate by double-sided tape. The thickness of the exfoliated Ge-H flakes was ≈4.5-4.7 μm which is shown in Figure 1C. Figure 1E shows an atomic force microscopy (AFM) image of the fabricated device structure, and Figure 1F shows the height profile of the device over the Si/SiO 2 substrate. The average area is 0.25 mm 2 . To create the contact pads, we used a copper transmission electron microscopy (TEM) grid (Ted Pella, Inc., 400 Mesh) during a thermal deposition of Au/Ti (95/5 nm). The TEM grid serves as a simple shadow mask and allows to create many contact pads on the flake surface without the need of a time-consuming lithography process. Note that the channel size between two gold pads is defined by the TEM grid mask (38 μm contact width × 26 μm channel width). Figure 2 presents the optoelectronic analysis of the fabricated photodetectors. To our best knowledge, this is the first study of solid-state Ge-H photodetector and its optoelectronic properties. The I-V curves measured on Ge-H crystal under dark and two illumination wavelengths are shown in Figure 2A, and for this purpose we used 365, 460, 530, 630, and 740 nm light. The I-V curves exhibit nearly linear behavior, confirming passable ohmic contacts between the metallic contacts and the flake. The power density for different wavelengths of light is fixed at ≈1 mW cm −2 . The Ge-H crystal has a clear photoresponse under different light illuminations. This agrees with the measured absorption edge of 1.85 eV for Ge-H and 1.97 eV for Ge-(CH 2 ) 3 OH, respectively ( Figure 6). The ON/OFF (photocurrent/dark current) switching ratio at 10 V bias was found to be in the range of 1.20-1.30 for different wavelengths. Figure 2B shows the I-V curve measured on Ge-(CH 2 ) 3 OH crystal. In contrast to the I-V curves for pure Ge-H in Figure 2A, the Ge-(CH 2 ) 3 OH crystal shows higher resistivity of about one order of magnitude. Additionally, the Ge-(CH 2 ) 3 OH exhibits more than two times higher photoresponse than the normal Ge-H as the ON/OFF switching ratio at 10 V is 3.75 for all illumination wavelengths. Furthermore, to quantitatively compare and evaluate the devices' photodetection performance as a function of light intensity, responsivity R is measured. Responsivity expresses the photocurrent generated per unit power of the incident light on the effective area and the formula is where I ON is current generated under illumination, I OFF is current measured in the dark, P is the incident light power density, and S is the effective area under illumination. The values presented here were measured in ambient, under 460 nm illumination at 10 V bias. The calculated responsivity values as a function of light power density are plotted in Figure 2C. For both samples (normal and modified), responsivity increases as the light intensity decreases, which is a typical feature of photodetectors based on layered materials. [20] The maximum responsivity, reached at 2 μW cm −2 light intensity, is 1157 and 362 mA W −1 for normal Ge-H and Ge-(CH 2 ) 3 OH, respectively. Significantly, those values exceed the highest responsivity reported for a photoelectrochemical (PEC) -based Ge-H photodetector by more than five orders of magnitude. [19] Another important figure of merit for photo detecting devices is photoresponse, defined as a ratio of The photoresponse for normal and modified Ge-H is plotted in Figure 2D   samples. Notably, the modified Ge-H flake exhibits higher photoresponse, indicating more efficient electron-hole pair generation by the incoming photons.
Detectivity D is another important parameter that describes the photodetector's capability to distinguish weak signals of light and can be calculated as below equation where R is responsivity, A active area, e the unit charge, and I OFF dark current. The maximum detectivity has been calculated as 3.03 × 10 9 Jones and 1.71 × 10 9 Jones, respectively, for Ge-H and Ge-(CH 2 ) 3 OH, as shown in Figure 3.
In addition to the ultrahigh responsivities, time-resolved photocurrent measurements shown in Figure 2E for Ge-H show rise and fall times smaller than 20 ms, which is the fastest response time reported of a Ge-H photodetector. Note that the time resolution of our probe station is 20 ms and it is therefore likely that the actual response times are even smaller than 20 ms. The cycle responsivity and stability were checked under alternating light ( = 460 nm) with a power intensity of 1 mW cm −2 at 10 V bias, as shown in Figure 2F. Based on the data, the device demonstrates stable ON/OFF switching behavior as there is little deviation in the cycles and the conversion between high conduction and low conduction is reversible.
Germanane (Ge-H) was prepared by topochemical deintercalation of CaGe 2 with hydrochloric acid at −20 C. Ge-(CH 2 ) 3 OH was prepared by post-functionalization of germanane utilizing Ge-H activation with equimolar sodium-potassium alloy (NaK) and crown ether (15-crown-5) system in anhydrous tetrahydrofurane (THF). [16b] Detailed experimental data are viewed in the Supporting Information. The materials were analysed by Fourier transform -infrared (FT-IR) and Raman spectroscopy (Figure 4).
X-ray photoelectron spectroscopy (XPS) was used to study valence state and bonding properties, Deconvolution of Ge 2p highresolution region revealed Ge-O peak at ≈1220 eV for both materials as well as Ge-H peak at ≈1217.2 eV for the Ge-H sample ( Figure 5D) and Ge-H/Ge-C at ≈1217.8 eV ( Figure 5B) for Ge-(CH 2 ) 3 OH sample. C 1s region revealed only adventitious carbon for the Ge-H sample ( Figure 5C), whereas Ge-(CH 2 ) 3 OH signal ( Figure 5A) could be deconvoluted into three curves representing Ge-C at ≈283.3 eV, C-O at ≈286.2 eV as well as C-C and adventitious carbon at ≈285 eV.
X-ray diffraction (XRD) analysis ( Figure 6A) of both germanane and Ge-(CH 2 ) 3 OH revealed reflection characteristic to the material, i.e., 002 and 004 reflections for germanane and 002, 004, 006, and 100 for Ge-(CH 2 ) 3 OH. Based on the 002 reflection, the interlayer distance is ≈5.5 Å for Ge-H and ≈11 Å for Ge-(CH 2 ) 3 OH. The width of the signals implies a higher degree of exfoliation causing an amorphous structure in the z direction. The bandgap of both Ge-H as well as Ge-(CH 2 ) 3 OH was determined from Tauc plot ( Figure 6B). Differential thermal analysis www.advancedsciencenews.com www.advopticalmat.de  (DTA, Figure 7.) of Ge-H revealed only the loss of hydrogen and residual water (≈1.5%) up to 300°C followed probably by decomposition of residual Ge-Cl bonds (≈2%) up to 600°C. On the contrary, Ge-(CH 2 ) 3 OH revealed not only a loss of water (m/z 18) and hydrogen, but also a loss of m/z 41, allyl cation C 3 H 5 + , which rep-resents a fragment of dehydration of -(CH 2 ) 3 OH moiety and this dealkylation step corresponds to ≈35 wt.%. The scanning electron microscopy (SEM) images clearly demonstrate a layered structure of both Ge-H ( Figure 8A) as well as Ge-(CH 2 ) 3 OH ( Figure 8B). Elemental mapping demonstrates   the presence of Ge and absence of C and O in the case of Ge-H ( Figure 9A) and the presence of Ge, C, and O for Ge-(CH 2 ) 3 OH ( Figure 9B). The elemental presence fits the initial analysed image.
The layered morphology of Ge-H was confirmed by transmission electron microscopy (TEM) analysis with well-visible Ge-H flakes (Figure 10A), and the selected area electron diffraction (SAED) shows the typical hexagonal pattern of Ge-H (inset of Figure 10A). After its functionalization, its crystalline structure was altered as evidenced by the amorphous SAED pattern in Figure 10B.

Conclusion
In this paper, we report for the first time a use of solid-state germanane as well as Ge-(CH 2 ) 3 OH photodetectors equipped with Au contacts and channel width of 26 μm 2 . The I-V characteristics of Ge-H photodetector clearly indicate a photoresponse with different wavelengths of light from 365 to 740 nm with the ON/OFF (photocurrent/dark current) switching ratio at 10 V bias of ≈1.11 to 1.30. The I-V curve measured on Ge-(CH 2 ) 3 OH crystal exhibits more than two times higher photoresponse than the normal Ge-H as the ON/OFF switching ratio at 10 V is 3.75 for all illumination wavelengths. For both, Ge-H as well as (3-hydroxypropyl) germanane, the responsivity increases as the light intensity decreases. The maximum responsivity, reached at 2 μW cm −2 light intensity, is 1157 and 362 mA W −1 for normal Ge-H and Ge-(CH 2 ) 3 OH, respectively. Significantly, those values exceed the highest responsivity reported for a PEC-based Ge-H photodetector by more than five orders of magnitude. The photoresponse increases with light power intensity for both samples. Notably, the modified Ge-H flake exhibits higher photoresponse, indicating more efficient electron-hole pair generation by the incoming photons. Both samples also show rise and fall times smaller than 20 ms, which is the fastest response time reported of a Ge-H photodetectors. The excellent performance of both photodetectors makes them very promising candidates for future device application.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.