Adam Rule

Karen Boyd
email: karenboyd@gmail.com

Jim Hollan
email: hollan@ucsd.edu

Aurélien Tabard
email: aurelien.tabard@univ-lyon1.fr

Restoring the Context of Interrupted Work with Desktop Thumbnails

Keywords: interruptions, memory, thumbnails, activitybased computing

Knowledge work is frequently interrupted. Interruptions enable collaboration and bring timely information, but they disrupt the fragile context of ongoing activities. Computers, now ubiquitous in knowledge work, have improved in their ability to track and restore digital context (documents and files), but they do a poor job of helping users restore mental context: the ideas, intentions, and motivations behind their work. Thumbnail images are an efficient way to help computer users refind documents; we ask if they can also be used to restore mental context. We tested how three manipulations to thumbnails of personal computer screenshots impact their ability to help viewers recognize past activities and recall accurate and detailed context. In a 2-week study we found that thumbnails of portions of the screen need to be larger than thumbnails of the entire screen for successful activity recognition and that static screenshots prompted more accurate contextual recall than animations.

Introduction

Knowledge work is frequently interrupted. One 13-month study found that knowledge workers switch activities once every 12 minutes (e.g. from writing a report to calling a customer) and that the majority of these activities (57%) are interrupted [START_REF] Mark | No task left behind?: examining the nature of fragmented work[END_REF]. This fragmentation is not necessarily bad. Interruptions bring timely information, foster collaboration, and direct attention to interesting or urgent work. But resuming interrupted activities takes time and energy. Early research estimated that programmers spend 15 minutes after each interruption "regaining concentration" [START_REF] Van Solingen | Interrupts: just a minute never is[END_REF]. Another field study by [START_REF] Iqbal | Disruption and recovery of computing tasks: field study, analysis, and directions[END_REF] confirmed that people spend a significant amount of time restoring their mental state after re-finding relevant papers and documents. Likewise, knowledge workers find it harder to restart interrupted tasks than to start new ones [START_REF] Czerwinski | A diary study of task switching and interruptions[END_REF].

Resuming knowledge work, whose non-routine problem solving activities routinely involves manipulating collections of information in external artifacts and internal memory, takes cognitive effort. While interruptions can make artifacts hard to find, physically or digitally obscuring them under the artifacts of intervening tasks, they are particularly disruptive to the contents of working memory. These memories get harder to reactivate the longer an inter-ruption lasts [START_REF] Altmann | Memory for goals: An activation-based model[END_REF] and their retrieval can be "blocked" if the interrupting activity is sufficiently similar to the suspended one [START_REF] Gillie | What makes interruptions disruptive? A study of length, similarity, and complexity[END_REF]. We refer to these memories as an activity's mental context. This context can be quite complex, especially for dynamic, creative, and non-linear activities like data analysis, programming, and writing. It often includes information that is difficult to represent externally or ephemeral such as intentions ("I'll call Bill next"), motivations ("this letter needs to sound more professional"), and ideas ("this graph may look better on a log scale").

Activity-Based Computing

Knowledge work increasingly involves computers, but computers have historically done a poor job supporting interrupted and interleaved work [START_REF] Bannon | Evaluation and analysis of users' activity organization[END_REF]. In the realm of restoring artifacts, switching tasks on a computer can take a flurry of finding, opening, and rearranging windows. Activity-Based Computing has attempted to address this issue by making it easier to manipulate groups of documents [START_REF] Bardram | Support for activity-based computing in a personal computing operating system[END_REF]. The paradigm can trace its roots back to Henderson and Card's Rooms work in 1986 and has seen a resurgence of interest in recent years [START_REF] Dumais | Stuff I've seen: a system for personal information retrieval and re-use[END_REF][START_REF] Kaptelinin | UMEA: translating interaction histories into project contexts[END_REF][START_REF] Karger | Haystack: A customizable general-purpose information management tool for end users of semistructured data[END_REF][START_REF] Rattenbury | CAAD: an automatic task support system[END_REF] but has yet to have a significant presence in mainstream operating systems. Regardless, this research only addresses half the problem. Computers provide even less support for what is often the harder part of restarting an interrupted activity: restoring mental context [START_REF] Iqbal | Disruption and recovery of computing tasks: field study, analysis, and directions[END_REF].

Visual Memory

One promising approach to restoring mental context was demonstrated by [START_REF] Cangiano | Capturing and restoring the context of everyday work: A case study at a law office[END_REF] who found that people who watched screen-recordings of past work were able to remember contextual details such as why they were working on a particular project or who they were talking to at the time. Humans have excellent visual memory. Participants shown 2560 images for 10 seconds each were able to correctly select which of two images they had seen before 90% of the time when tested two days later [START_REF] Standing | Perception and memory for pictures: Single-trial learning of 2500 visual stimuli[END_REF]. A more recent study found that subjects were able to correctly identify the image they had seen before 87% of the time when the only difference between the images was the pose of an object, such as the arrangement of beads on an abacus [START_REF] Brady | Visual long-term memory has a massive storage capacity for object details[END_REF]. Cangiano et al. demonstrated that humans can do more than discriminate; they can also attach complex meaning to images, even passively taken ones. [START_REF] Sellen | Do life-logging technologies support memory for the past?: an experimental study using sensecam[END_REF] confirmed this result, finding that images taken automatically by a wearable camera cued as detailed of episodic memories as images taken by actively pressing a button. Passively recording and then visualizing knowledge work may ease the burden of resuming interrupted activities by helping people restore mental context that was never externalized in artifacts.

Thumbnails

Research on visualizing and recognizing computer activity has focused on website thumbnails. [START_REF] Kaasten | How people recognise previously seen Web pages from titles, URLs and thumbnails[END_REF] showed that people could reliably (80% of the time) identify a website domain they had previously visited (e.g. www.cnn.com) using a 132 x 132px thumbnail image of that website and could identify its exact topic (e.g. a CNN article on the 2008 election) with a 208 x 208px thumbnail. Moreover, [START_REF] Teevan | Visual snippets: summarizing web pages for search and revisitation[END_REF] found that people were able to re-find previously visited websites more quickly using thumbnails than page titles.

But, these studies are limited to web-browsers and the recognition of well-defined pages. While useful, they do not address the complex nature of real-world work that typically involves not only multiple applications but also dynamic, creative, and non-linear activities. Our work expands on this research by asking if thumbnails can be used to represent not only documents, but also whole activities and their mental context.

Research Question & Hypotheses

Q: How do the cropping, animation and rehearsal of a thumbnail showing a computer desktop impact its ability to help people recognize past activities and recall accurate and detailed context.

Cropping

Small thumbnails that show a full desktop can be ambiguous; windows are tiny, text is unreadable, and it is hard to see the user's cursor. Keeping the thumbnail the same size but having it show only a small portion of the screen, such as the area around a user's cursor, may support better activity identification as users see one region in detail and may be able to mentally "fill in" the rest from memory (Figure 1).

Figure 1: Cropped thumbnails (B) can be the same size as full-screen thumbnails (A) but show less of the screen H1: People will recognize activities with smaller thumbnails if those thumbnails show a cropped portion of the screen. By forcing people to "fill-in" screen content, cropped thumbnails will cue less accurate but more detailed memories than full-screen thumbnails as people search memory for context rather than read it off the thumbnail.

Animation

By displaying activity over time, animations provide more information than individual screenshots, potentially helping users distinguish similar activities and retrieve more specific memories. However, making sense of animations takes focus and may cause viewers to neglect context held in memory [START_REF] Tversky | Animation: can it facilitate[END_REF].

H2: Animations will cue more accurate but less detailed memories than screenshots.

Rehearsal

Rehearsing memories makes them easier to recall, but can also change their contents [START_REF] Hupbach | The dynamics of memory: Context-dependent updating[END_REF]. Asking people to reflect on particular moments may make those moments easier to recall, but can also restrict the number of details they remember about them.

H3: Memories for moments that have been rehearsed will be more accurate but less detailed than those for unrehearsed moments.

Methods

Participants and Materials

Six graduate students (4 female, ages 23-29) recorded their workday (Mon-Fri) computer activity for two weeks using a modified version of Selfspy 1 , an open-source key-logger. The computers used for recording included stand-alone laptops, laptops connected to an external monitor, and all-inone desktops. The tool took a screenshot every time a participant clicked or typed, and every 30 seconds while their computer was awake but idle. All screenshots were stored on an SD card. Participants received $50 in gift cards.

Procedure -Recording

Participants could pause the recording at any time and were instructed to do so whenever they used their computer to communicate with someone outside the study. Every 30 minutes, Selfspy showed the participant a recent screenshot and asked them "What are you doing?" Participants could ignore these experience samples, but were encouraged to fill out as many as possible. At the end of each day, participants were asked to provide additional detail on up to five samples from earlier that day. While viewing each sample's screenshot and textual description participants answered the following questions: Work by [START_REF] Brandt | txt 4 l8r: lowering the burden for diary studies under mobile conditions[END_REF] has shown that asking users to write short descriptions in the moment and fill in details later reduces the impact of experience sampling interruptions but maintains response quality.

Procedure -Testing thumbnails

Participants attended a one-hour lab session at the end of each week to review up to 40 thumbnails representing moments from the prior week. The thumbnails varied along three conditions, meaning we tested 8 variations in a 2x2x2 design:

A. Screenshot or animation B. Full screen or cropped C. Rehearsed moment (e.g. described in detail at the end of a day earlier that week) or unrehearsed moment (randomly selected)

We tested the four types of screenshot thumbnail during the first week's review (i.e. screenshot condition with BxC). Similar to [START_REF] Kaasten | How people recognise previously seen Web pages from titles, URLs and thumbnails[END_REF], when the participant pressed a start button, a 20px high thumbnail would appear.

(The width of the thumbnail depended on the recording computer's screen ratio). Every two seconds the thumbnail grew 20px taller and proportionately wider until the participant recognized the represented activity and pressed a button to stop the growth. Full screenshots showed the participant's entire screen whereas cropped screenshots showed an expanding area around the user's mouse. Participants then answered the three questions used in the end-of-day debriefs and moved on to the next thumbnail. We tested the four types of animation thumbnail in the second week. Each animation showed a time-lapse of five minutes of computer activity played at 5x normal speed. Full animations showed the participant's full screen at the native resolution of the recording computer, while cropped animations showed a 520px high area around the participant's cursor location at the time of recording. (520px was the height at which our first four participants could recognize 80% of the activities in their week 1 cropped screenshots). Participants pressed a button to start each animation, pressed a second button once they recognized the represented activity, and then filled out the debrief questions before moving on to the next thumbnail.

Two participants had too few debriefed experience samples in the second week to include their data. In total, we collected 145 responses to screenshots and 70 to animations.

Measures

We measured participant's responses in six ways:

1. Thumbnail size (for static thumbnails) or duration (for animations) at the point of activity recognition 2. Self-rated Memory Strength and Thumbnail Appropriateness 3. Memory Accuracy 4. Episodic Detail 5. Event Specificity 6. Time Discussed

We coded each response for measures 3-6. Memory Accuracy was coded on a scale of how well the activity described in the review matched the activity described in the end-ofday debrief. The levels included 0) no match, 1) partial match, and 2) mostly matches. Accuracy could only be coded for rehearsed thumbnails since unrehearsed thumbnails had no end-of-day debrief responses for comparison. For Episodic Detail, we recorded the number of contextual details shared about an activity including who, what, where, when, why and feeling information. For Event Specificity, we recorded the number of events described in the response at the Action (i.e. "copy and pasting"), Activity ("editing this discussion slide"), or Project ("working on my presentation") level. Lastly, we coded whether the text mentioned events that took place in the Past, Present, or Future relative to the time the thumbnail was taken.

Two authors (AR and KB) coded the responses to the end-of-day debriefs for the first week of the study and iterated the coding rubric until they achieved a Cohen's Kappa of >0.60 for each category. They then separately coded the thumbnail responses.

Results

We fit a mixed linear model to each measure, using animation, cropping, and rehearsal as our predictor variables. Significant effects were detected by removing individual predictors from each model and using a one-way ANOVA to test for differences between the full and reduced models.

Size of Screenshot Thumbnails

Participants reliably (80% of the time) recognized their activity when full screenshots thumbnails were 320px tall (x̄ =240px, σ=154px) and when cropped screenshot thumbnails were 460px tall (x̄ =370px, σ =210px). For our participants, these heights correspond to thumbnails that were 30% and 45% the height of their screen, respectively. This difference was significant (χ 2 =23.50, df=2, p<0.001). Rehearsing, however, did not have an effect on the required thumbnail size (x̄r=306px, x̄u=305px, χ 2 =1.79, df=2, p>0.05).

Duration of Animation Thumbnails

Participants reliably (80% of the time) recognized their activity before the 9.5 second mark of full-screen animations (x̄=8.0s, σ=4.1s) and the 14.5 second mark of cropped ani-mations (x̄=8.1s, σ=5.3s). This difference was not significant (χ 2 =1.79, df=2, p>0.05) nor was the difference between rehearsed and unrehearsed thumbnails (x̄r=8.2s, x̄u=7.9s, χ 2 =1.91, df=2, p>0.05).

Memory Strength, Thumbnail Appropriateness, and Accuracy

Participants tended to rate memories cued by animations as stronger than those cued by screenshots (x̄a=1.53, x̄s=1.26, max=2, χ 2 =8.87, df=4, p=0.064) but gave similar ratings across the cropping and rehearsal conditions. Thumbnails were given similar ratings of appropriateness across all conditions. However, memories were more accurate when cued by screenshots than animations (x̄s=1.70, x̄a=1.27, max=2, χ 2 =11.628, df=2, p<0.01).

Episodic Detail

Participants were remarkably consistent across conditions in the number and type of details they shared in their responses. While responses to animations were significantly shorter (x̄a=34.1 words, x̄s=61.0 words χ 2 =16.68, df=4, p<0.01), they were equally likely to include each of the six types of contextual information we tracked. Figure 2 shows the frequency of each type of contextual information across all conditions.

Event Specificity

Participants were consistent across conditions in the abstractness of events they described. Of those cases where participants mentioned multiple levels of events, such as talking about an activity and then describing the individual actions that made up that activity, they were much more likely to start with high-level events and then describe lower-level ones (as happened 71% of the time) than to go from describing low-level events to high-level ones (28% of the time) (χ 2 =21.6, df=1, p<0.001). Thus statements of the form "Here I was trying to find car insurance again [high], trying to get a quote from a different company [low]" were more common that those of the form "I'm copying labels in the dictionary [low] so that I can make a count of how many certain types of signs there are in the sign language and handshake dictionary [high]".

Time Discussed

Participants were also consistent across conditions in how often they discussed past events (29% of responses) or future events (24%). Nearly half (47%) of responses included a reference to either past or future events. Of those cases where multiple time periods were mentioned, participants were equally likely to move forward or backward in time (χ 2 =0.096, df=1, p=0.757).

Discussion

Cropping

Cropped thumbnails had to be larger than full-screen thumbnails to cue recognition of past computer activity, but there was no difference in the accuracy or detail of the memories they cued. This evidence does not support H1.

Looking at how participants used thumbnails helps explain why. In some cases a small cue such as a unique photo brought back a set of memories. For example, upon seeing a 160px high cropped screenshot showing part of a gorilla, one participant recognized that they were "watching a webpage to look up some information about Koko the gorilla and his vocalization abilities." But more often, participants pieced together what they were doing from multiple, distributed cues. For example, in the following response, the participant uses the fact that two windows were open simultaneously to remember a past meeting, why they had that meeting, and what they planned to do afterwards. "Here I have the Github page for DIVY open and I also have the Terminal window open so this is when I was meeting with John… so I could try out some of my data clustering with this code to see if it works for me and then we could show it to the Smith lab." The cropped screenshot thumbnail that cued this response had to expand to be 640px high before the participant recognized their activity.

We also found that the mouse is not a good measure of attention. Often the mouse was over a blank or unremarkable portion of the screen so participants had to wait a long time before the cropped thumbnail included relevant information.

Animation

Participants tended to be more confident of memories prompted by animations but these memories were surprisingly less accurate. Animations prompted shorter responses but these included as much contextual information (who/what/where/when/why/feeling) as responses prompted by static screenshots. This evidence does not support H2.

One cause of this mismatch between perceived and actual accuracy was that in several cases participants simply described what was happening in an animation and ignored mental context. For example, compare the following end-ofday debrief and review text cued by an animation.

Debrief: "So I released a batch of twenty participants through Mechanical Turk this morning at 9am and for some reason the traffic was very slow today. So until 2pm I think I had only gotten about ten people. But then I didn't want to wait until I get all twenty so I went ahead and analyzed the data just to see the pattern and saw that it wasn't in the direction that I wanted."

Review: "So I was making a pivot table and I was trying to see if I saw any pattern from my data, but I'm not sure if it was before I got all my data or after."

The debrief focuses on motivating events and talks about the activity abstractly, "analyzed the data", whereas the review text is much more specific, "making a pivot table", but fails to mention the larger context.

Rehearsal

We found no difference in the memories cued by rehearsed and unrehearsed thumbnails. Several factors could have led to this result. First, there may have been too much time between debriefing and reviewing. Whatever memory benefit the rehearsal provided may have dissipated by the time of review. Alternatively, our participants, all graduate students, may have been working on too few projects for rehearsing to have a targeted effect. Since graduate students usually work on a few large projects, letting them rehearse five memories from the day may have effectively let them reflect on most of their projects. This evidence does not support H3.

Content of Memories

A large proportion of responses described why an activity was taking place (47%), who was involved (40%) and when it occurred (38%). Some explanations of why an activity occurred were quite complex, describing multiple, conflicting motivations or a sequence of actions that depended on the current action. Descriptions of when an activity took place were often relative such as "before starting my day" or "during lab meeting." Absolute time (e.g., 3pm) was rarely mentioned, though participants did use the computer clock in thumbnails to distinguish similar activities or confirm guesses, such as "Oh, yep. Tuesday morning. That would make sense."

To check if these contextual details were recalled from memory or could have been read directly from the thumbnail we conducted a post-hoc analysis. On average, responses included 4.47 (σ =1.89) episodic details and of these, 1.95 (σ =1.60) seemed to be reconstructed from memory. Specifically, the majority of feeling information (98%), why (81%), and where (78%) information seemed to be reconstructed from memory whereas when (47%), who (41%) and what (34%) information were more likely to have been represented directly in the thumbnail.

Participants mentioned activities in most of their responses (86%) such as "going through and finding references from other references" or "trying to find pictures of my two advisors". They also mentioned actions in most responses (56%), but these were often just descriptions of what was on the screen, "I see that I was sending an email to someone".

Participants also frequently mentioned events leading up to or after the thumbnail (47% of responses). Many of the past events were motivation, and while many of the future events included motivations, a number were outcomes "this isn't the way I ended up solving [this problem]."

Limitations

Given privacy concerns with recording computer activity, we only recruited 6 participants, who provided 215 responses. These participants were graduate students whose work often involves a small number of projects. Other working styles could have produced different results. We also did not balance presentation of screenshots and animations across the two weeks of our study, so the differences between screenshots and animations may be an artifact of having an extra week to practice giving responses.

Implications for Design

Thumbnails of full-screen images

Without a better predictor of users' attention than the mouse (e.g. gaze), full-screen thumbnails can be smaller than cropped thumbnails and covey as detailed of memories.

Our results also suggest that thumbnails still work for recognizing cross-application activities, but need to be larger (320px high for 80% recognition) than those used for recognizing website (208px high according to [START_REF] Kaasten | How people recognise previously seen Web pages from titles, URLs and thumbnails[END_REF]). This result is encouraging since a website thumbnail is a summary of one window, but a desktop thumbnail contain more content (overlapping windows, toolbars, etc.). This size increase may also be an artifact of computer monitors increasing in pixel density over the last decade.

Animations are less effective than expected

We found that animations, at least 5x time-lapses, cue less accurate memories than screenshots and found no evidence that they produce memories that are any more detailed. Showing activity over time may be better accomplished with small-multiples or thumbnails that allow scrubbing.

No need for rehearsing

We did not find evidence that random experience sampling and rehearsing improved memory in our tasks. This is in line with [START_REF] Sellen | Do life-logging technologies support memory for the past?: an experimental study using sensecam[END_REF] finding that passively taken images were as good of triggers for remembering past events as actively taken images. This does not diminish the value of bookmarking past events to create landmarks for future reviewing, but one should not expect these events to be more memorable than others.

Conclusion

Existing computer systems do a poor job of helping users restore the mental context of interrupted activities. This restoration is particularly challenging for dynamic, creative, and non-linear activities that span multiple applications. Given humans' excellent visual memory, we investigated how visual thumbnails of computer desktops could help restore the memories that make up this context. Our work extends the scope of previous thumbnail research from rep-resenting previously visited websites to representing crossapplication activities and their context.

Our two-week field study of six graduate students confirmed that thumbnails cue a significant amount of contextual information. We compared eight types of thumbnails: full screenshots, cropped screenshots, full animations, and cropped animations (each with or without rehearsal). Across conditions, a non-trivial proportion of thumbnail responses included information about what activity was taking place (99%), why it was happening (47%), who was involved (40%), and when it happened (38%). Between conditions, we found that 80% of activities could be recognized with full screenshot thumbnails that were 320px high, whereas thumbnails cropped around a region of interest (cropped screenshots) needed to be 460px high. Animations cued more targeted but less accurate memories than screenshots, which is surprising as we expected that seeing the activity unfold would lead to more accurate memories. Rehearsal had no discernable impact on the accuracy or detail of recalled context.

There is a rich design space to be explored in future research. For example, we plan to explore composing activityspecific thumbnails from snippets of multiple application events (e.g., a series of movements between applications related to accomplishing a specific activity) rather than focusing exclusively on thumbnails of a specific screen location at a specific moment in time. Animations may be more useful in showing these tightly coupled sequences. Another important next step will be to document how the contextual information cued by thumbnails is actually used to restart interrupted activities, as the use of this information will inform how it should be represented.

Our findings support the notion that visual cues can be used to recall the detailed context of complex activities. This finding is important because recovering context is a key first step towards restarting interrupted activities.

2.

  How do you know these details? (I remember this moment exactly, I know from experience, or I'm guessing) 3. How well does this image represent what you were doing? (Very well, Somewhat, or Not at all) These responses gave us a detailed baseline description of what participants were doing at the time of the sample.

Figure 2 :

 2 Figure 2: Participants recalled why, who, and when context in a large proportion of responses

Acknowledgments

This research was funded by NSF grant #1319829

://github.com/activityhistory/selfspy


Joel S Pendery

Olivier Merchiers

Delphine Coursault

Johan Grand

Habib Ayeb

Romain Greget

Bertrand Donnio

Jean-Louis Gallani

Charles Rosenblatt

Nordin Félidj

Nordin Felidj

Yves Borensztein

Emmanuelle Lacaze

Gold nanoparticle self-assembly

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

It is now established that a large number of well-defined hybrid systems composed of liquid crystals (LC) and nanoparticles can be prepared, either to vary the properties of the LC or those of the nanoparticles [START_REF] Bisoyi | Liquid-crystal nanoscience: an emerging avenue of soft selfassembly[END_REF][START_REF] Lagerwall | A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio-and microtechnology[END_REF]. On the other hand, it is also well-known that optical properties of 0g Univ Strasbourg, Institut de physique et de chimie des matériaux de Strasbourg (IPCMS), CNRS, UMR7504, F-67034 Strasbourg 2, France the nanoparticles can be tuned if controlled self-assemblies of nanoparticles can be prepared. This calls for a better understanding and control of these hybrid systems. For example, Quantum dots emission can be modified through well-controlled self-assemblies [36, 35, ?]. In addition, the localized surface plasmon resonance (LSPR) properties of metallic nanoparticles may be greatly varied by the coupling between nanoparticles [START_REF] Myroshnychenko | Modelling the optical response of gold nanoparticles[END_REF]. In particular, formation of linear self-assemblies is highly desirable since it exhibits anisotropic optical properties which can be activated by external parameters, like light polarization [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF], similarly to oriented nano-rods for example [START_REF] Chu | Electrically controlled surface plasmon resonance frequency of gold nanorods[END_REF][START_REF] Liu | Self-alignment of plasmonic gold nanorods in reconfigurable anisotropic fluids for tunable bulk metamaterial applications[END_REF].

In the case of nanoparticles dispersed in thermotropic LCs, two main mechanisms have been identified, which facilitate control of the nanoparticles self-assembly by the LC matrix. First a segregation process leads to a localization of the nanoparticles at the LC/air interface. For cholesteric LC films (CLCs), the obtained nanoparticle assemblies can mimic the well-known cholesteric fingerprints [START_REF] Bitar | Cholesteric liquid crystal self-organization of gold nanoparticles[END_REF]. Second, trapping phenomena have been demonstrated for systems with topological defects, such as saturn ring defects surrounding micrometer sized colloids [START_REF] Senyuk | Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals[END_REF], blue 1 phase disclinations [START_REF] Kikuchi | Polymer-stabilized liquid crystal blue phases[END_REF][START_REF] Karatairi | Nanoparticle-induced widening of the temperature range of liquid-crystalline blue phases[END_REF][START_REF] Rožič | Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation[END_REF][START_REF] Senyuk | Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals[END_REF], screw disclinations of twist-grain-boundary phases [START_REF] Cordoyiannis | Different modulated structures of topological defects stabilized by adaptive targeting nanoparticles[END_REF] or linear patterns of smectic edge dislocations [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF][START_REF] Milette | Reversible long-range patterning of gold nanoparticles by smectic liquid crystals[END_REF]. In the presence of topological defects, nanoparticles are viewed as particulate impurities that distort the LC in their vicinity, or induce disorder. Located in topological defects, they take the place of a portion of the already disordered defect cores and allow for a decrease of the energy within the system [START_REF] Kikuchi | Polymer-stabilized liquid crystal blue phases[END_REF]. When the defects are linear, like in smectic oily streaks dislocations, single chains of nanoparticles can be obtained exhibiting highly anisotropic LSPR [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF]. It now becomes important to identify if new kinds of topological defects can be oriented. Two questions can thus be asked: First, are other kinds of topological defects appropriate for a controlled and oriented self-assembly of nanoparticles? Second, how the nature of the trapping sites controls the self-assembly structure, in particular the spacing between nanoparticles which, in return, controls the optical response of the hybrid system associated with gold nanoparticles?

The goal of the present paper is to study gold nanoparticle self-assembly based on the nanoparticles dispersion in a CLC matrix distorted by competing anchorings at the substrate/CLC and CLC/air interfaces. These competing anchorings are known to produce a 90 • rotation of the cholesteric helix in the vicinity of the CLC/air interface [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF]. Below the area associated with the 90 • rotation, the cholesteric helix is perpendicular to the substrate. Above this area, the helix is parallel to the CLC/air interface, its orientation being linked to the ratio e/p, with e the thickness and p the cholesteric pitch [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF]. A rotation of the helix parallel to the CLC/air interface can consequently be induced when the pitch is varied, for example by irradiation of cholesterics doped by photoactive chiral dopants [START_REF] Eelkema | Nanomotor rotates microscale objects[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF]. However the intimate structure of the area associated with 90 • rotation of the cholesteric helix is not known. The electromagnetic coupling between gold nanoparticles of diameter 4.2nm trapped in the distorted CLC matrix has been evidenced through the measurements of a welldefined anisotropic LSPR. In a second step, the majority of the CLC matrix has been eliminated by evaporation, revealing nanoparticle self-assemblies with a needle shape. Analysis of LSPR data using calculations within the frame of generalized Mie theory allows to interpet both data, with and without CLC. They both appear to be consistent with the presence of nanoparticle needles. Whereas these hybrid systems lead to the formation of only slightly anisotropic aggregates for large nanoparticles [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF] the balance between aggregation and trapping phenomena turns in favor of trapping for small nanoparticles. Moreover the comparison with LSPR data obtained with smectic topological defects or nanoparticle deposition on the substrate without liquid crystal evidences a shorter internanoparti-cle distance in CLC matrix. Interpretation of these data suggests that the presence of CLC topological defects may induce the trapping of nanoparticles and the formation of needles, but within a defect core of typical diameter smaller than the nanoparticle size. The results demonstrate a direct relation between nanoparticles spacing and nature of liquid crystal traps. They demonstrate additionally that the study of nanoparticle self-assembly processes can reveal information about the intrinsic deformations of the LC film.

Materials

Polyvinyl alcohol (PVA) was spin coated onto cleaned glass slides, baked at 120 • C to polymerize, and manually rubbed unidirectionally with cotton to promote planar anchoring at the surface. A cholesteric liquid crystal (CLC) mixture was prepared by doping 2wt.-% of the chiral agent S-2011 (MLC-6247 from Merck) in the nematic liquid crystal E7 (Merck). This sets the helical pitch at p = 5.5µm. E7 is composed of the mesogens 5CB (4'-pentyl-4-cyanobiphenyl), 7CB (4-heptyl-4'cyanobiphenyl), 8OCB (4-octyloxy-4'-cyanobiphenyl) and 5CT (4-cyano-4"-pentylterphenyl) and is in the nematic phase at room temperature with the phase diagram of K(-20 • C)N(58 • C)I. Here K, N and I stand for the crystalline, nematic and isotropic phase respectively. It has a high optical anisotropy ∆n = n en o = 0.23, where n e = 1.75 is the extraordinary index, parallel to the optical axis and n o = 1.52 is the ordinary index, perpendicular to the optical axis.

The CLC mixture was spin coated onto the rubbed PVA-coated glass to give an average thickness of ∼ 12µm. Inspection by optical microscopy showed a striped texture, of period Λ, of the cholesteric hybrid sample (Figure 1a). Here the PVA substrate promotes planar anchoring and the CLC helical axis, at the substrate and within the bulk, is perpendicular to the substrate. At distance around p/2 below the CLC/air interface, the helical axis undergoes a rotation to change from perpendicular to the substrate in order to better accommodate the director's preferred homeotropic boundary condition [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF] (Figure 1b). Stripes are observed by optical microscopy and are associated with the helical axis parallel to the CLC/air interface. They are perpendicular to the helix axis and of period slightly larger than two times the cholesteric pitch [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF].

Gold nanoparticles have been prepared following the "inverted Brust-Schiffrin" method, as described by Li et al. [START_REF] Li | Mechanistic insights into the brust?schiffrin two-phase synthesis of organochalcogenate-protected metal nanoparticles[END_REF]. 1.4mL of a 0. TOAB in toluene (TOAB is trioctadecylammonium bromide), under vigorous stirring at room temperature. After 1 hour, 2mL of freshly prepared aqueous solution (3 mmol) of NaBH 4 are added. After 10 minutes, 2mL of a 0.6mmol solution of dodecanethiol in toluene are added. The reaction medium is left under stirring for 20 hours. The organic phase is kept, washed once with 500mL of a 2 percent sulfuric acid solution, and then three times with 500mL of pure water. The organic phase is then centrifuged to eliminate the larger aggregates. The supernatant is kept and evaporated under vacuum down to a volume of a few mL.

It is poured into an excess of ethanol. A light centrifugation is performed (2 minutes at 2000rpm), the supernatant is discarded, the precipitate is dried under vacuum. Gold nanoparticles, 4.2nm in diameter (polydispersity 18 percent) were then suspended in toluene. A solution of concentration of 500µg/mL was used. Deposition of 10µL of the suspended nanoparticles solution was conducted on a hot CLC sample, at 55 • C to preserve the striped texture but to expedite the evaporation of the solvent. After the toluene evaporated, the sample was removed from the heating stage and the CLC texture was observed by optical microscope to confirm the absence of any modification to the period of the stripes. This demonstrates that the CLC pitch was not affected. We can see the presence of nanoparticles by a change of colour of the striped areas larger than 500µm × 500µm. These were the only modifications to the striped textures we observed. This observation allowed for localized measurements of the LSPR of the hybrid system, CLC/nanoparticles. Extinction spectra were measured with an optical microscope using a 50x objective lens, with a numerical aperture of 0.55. The sample was inverted such that the polarized light is incident on the CLC/air interface. The transmitted light was collected and then dispersed using a LOT Oriel MS260i spectro-photometer. A 2mm pinhole allowed an area of 100µm × 100µm to be probed. To obtain only the extinction spectra of the nanoparticles, the spectrometer data of the CLC/nanoparticle hybrid sample for a given area and stripe orientation was referenced to a CLC sample with a similar thickness and stripe orientation. Data were taken for incident light polarizations parallel and perpendicular to the stripes.

Results

LSPR of nanoparticles/CLC hybrid films

The extinction spectra of Figure 2a were obtained on the CLC/gold nanoparticle film with stripes perpendicular to the anchoring direction. The experimental data were fitted with a Gaussian curve. Spectra in Figure 2a display a plasmon resonance of λ para = 568nm when the light is polarized parallel to the stripes and λ perp = 550nm when the light is polarized perpendicular to the stripes. An extinction spectrum was measured for isolated gold nanoparticles in toluene (see Figure 2b) and the plasmon resonance peak was 516 ± 1nm. This gives a red-shift for the CLC/gold nanoparticles hybrid film of 52nm for light polarized parallel to the stripes and 34nm for light polarized perpendicular to the stripes, exhibiting an anisotropy in the LSPR with respect to the stripes orientation. However the LSPR anisotropy appears to vary from place to place. In fact, LSPR anisotropy is related to anchoring orientation. For stripes oriented parallel to the anchoring direction (5 of the 15 measured zones), the larger red-shift occurs in majority for polarization perpendicular to the stripes and thus for polarization perpendicular to the anchoring direction (4 of the 5 measured zones of stripes parallel to the anchoring direction). This is also true for stripes perpendicular to the anchoring direction. For these 10 measured zones, the red-shift is larger for polarization parallel to the stripes. If we consider only stripes perpendicular to the anchoring directions, we ob-tain λ para = 565 ± 3nm and λ perp = 555 ± 5nm It is well known that the localized surface plasmon resonance of a metal nanoparticle is highly dependent upon the optical index of the surrounding media [START_REF] Kelly | The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment[END_REF]. However, for the case of metal nanospheres in a liquid crystal, Park and Stroud [START_REF] Park | Splitting of surface plasmon frequencies of metal particles in a nematic liquid crystal[END_REF] have shown that the LSPR is weakly dependent on the orientation of the electric field along the ordinary or the extraordinary direction. By using their model, we calculated an anisotropy of the LSPR of nanoparticles in CLC of only 2nm, against a maximum of 18nm observed experimentally (Figure 2a). Moreover, the obtained red shifts for the nanoparticles in the CLC with respect to the LSPR of the isolated nanoparticles in toluene, whose index of refraction is n tol = 1.498, is calculated to be of only 8 and 10nm, along the ordinary and extraordinary directions, respectively. Such an effect therefore does not account for the large observed red-shifts. On the other hand, it is also known that the electromagnetic interaction of close particles can lead to large redshifts (e.g. for polarisation along a chain of particle, or for two-or three-dimension aggregates). The observed red-shift, not explained by the index of refraction of the CLC, therefore indicates that aggregates of nanoparticles are likely present within the CLC matrix. As shown by the optical anisotropy, their shape and structure may be directly induced by the cholesteric matrix. To understand how the self-assembly process may occur, the aggregate's shape and structure have to be elucidated.

AFM and LSPR after CLC evaporation

To resolve the aggregate shape utilizing an atomic force microscope (AFM), the sample was placed in a vacuum oven to remove a significant portion of the liquid crystal. This evaporation was performed within a vacuum at 10 -7 bar for two days and then heating the sample to 40 • for an additional three days to expedite the evaporation process. The sample was removed from the vacuum and the surface was mapped using optical microscopy (see Figure 1c). Pictures of the entire surface were taken using a Leica DMRX optical microscope with an objective lens magnification of 5x and then stitched together (Figure 1c). Not all of the CLC was evaporated, which is evident by the presence of the dark, divergent lines on the map corresponding to thick CLC regions. This was expected since evaporation of large liquid crystal molecules is difficult. However, using this panoramic surface map, regions with only a thin CLC layer could be selected for AFM imaging (Figure 3c). In addition, these same areas were used for LSPR measurements (Figure 4). Both regions with and without a plasmon resonance were probed using the AFM. Regions of 100µm × 100µm, of size similar to the one measured by spectro-photometry, were imaged and zooms were performed until individual aggregates could be resolved. No particles could be imaged in regions without a plasmon resonance, in agreement with having the number of nanoparticles below the threshold for LSPR measurement. In areas that exhibited a LSPR, either a homogeneous AFM topography and phase were present or areas with needle-like structures were observed (Figure 3a and3b). The needles, absent in the regions where no LSPR is measured, are therefore the nanoparticle aggregates, as it will be discussed in detail below. If we concentrate on Figure 3a, two distinct regions are observed with needles: 1) the valley area on the left half of the image and 2) the large, elevated region on the right half of the image. In the topography, flat needle-like aggregates are clearly visible, but only in the lowest region [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF]. In contrast, in the phase data (Figure 3b), the needle shaped aggregates are visible in the lowest region but also appear, less clearly but in larger quantity, in the highest region [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF]. The length of the needles varies from 100 to 400nm while the width varies from approximately 30 to 40nm. Taking into account the curvature radius of the AFM tip, varying typically between 5 and 30 nm [START_REF] Rodriguez | Probing the probe: Afm tip-profiling via nanotemplates to determine hamaker constants from phaseâ"distance curves[END_REF], the needles width can not be precisely determined. It may range from 1 to 5 nanoparticles. The topography of the needles investigated in the (1) valley (Figure 3c profile 1) shows a height of approximately 2 -3nm for the needles. Since the diameter of a nanoparticle is 4.2nm, one must conclude that the nanoparticle needles are flat on the surface, one particle thick but still partially submerged in the CLC. In addition, around the nanoparticles, in the lowest (1) as well as in the elevated (2) region, the AFM phase image is similar (Figure 3b). The phase is correlated with the stiffness of a given region [START_REF] Magonov | Phase imaging and stiffness in tapping-mode atomic force microscopy[END_REF], and it suggests that both of these regions are covered in CLC. Accordingly, the AFM phase on top of the nanoparticles is higher since nanoparticles consist of harder material. This is evident in the lowest area [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF] where the AFM tip directly interacts with the nanoparticles. This is also the case in the most elevated region (2), although less clearly, in agreement with needles covered by CLC, as shown by topography data, with no protuberant needles. These needles, after CLC evaporation, get close to the air/CLC interface and modify its stiffness. This allows us to observe the needles using AFM. These results finally reveal the presence of needles mostly covered by CLC, which may be responsible for the LSPR signal (Figure 4), measured over the same area as the AFM data (Figure 3).

The region at the bottom left of the image is believed to correspond to a demixing of E7 liquid crystal induced by the evaporation, leading to a crystallized hard area, as shown by phase image. It may correspond to pure 8OCB, pure 5CT or a mixture of the two, which are expected to be crystalline at ambient temperature.

Measurements of LSPR band performed on 7 areas of the evaporated sample mainly lead to similar LSPR band (Figure 4), parallel (x-axis λ = 564 ± 1nm) and perpendicular (y-axis λ = 566 ± 1nm) to the anchoring direction. The homogeneity of the LSPR signal suggests that the needles can be present but covered by a CLC film too thick to allow their observation with AFM. This is the case only when fully homogeneous topology and phase data are obtained with AFM. The absence of anisotropy, in contrast with the results obtained previous to evaporation, is in agreement with the observation by AFM of disordered needles. We will show in the following that formation of needles not only allows one to interpret the data after evaporation, but also allows one to interpret the optical properties in hybrid nanoparticles/CLC films, strongly suggesting that aggregation phenomenon is mostly independent of the evaporation process. The small size of the needles is consistent with the absence of any change of texture for the hybrid nanoparticle/CLC films observed by optical microscopy compared to pure CLC. A smaller disorder of the needles orientation may be present in the CLC/nanoparticle hybrid sample, the CLC evaporation process modifying mainly the orientation of the needles.

LSPR analysis

To interpret the LSPR data after removal of the cholesteric film, we associate the needles with the presence of nanoparticle ribbons of disordered orientation.

We have considered ribbons formed by hexagonally packed nanoparticles, characterized by the inter-nanoparticles edge-to-edge distance, the so-called spacing, s. To fit with the AFM data, we have considered long 2D ribbons only and calculated the related LSPR by the generalized Mie theory [START_REF] Mackowski | Calculation of total cross sections of multiple-sphere clusters[END_REF]. The optical index of the embedding medium has been taken equal to that of alkylthiols, (n=1.458). Due to the small distance between the nanoparticles, there will be mainly alkylthiol molecules between them. It is well-known that in nanoparticle chains, for a given nanoparticle spacing, the red-shift of the LSPR wavelength with respect to isolated nanoparticles, increases with the length of the chain until it saturates [START_REF] Khlebtsov | Optical properties and biomedical applications of plasmonic nanoparticles[END_REF][START_REF] Yockell-Lelièvre | Coupling of localized surface plasmon resonance in self-organized polystyrene-capped gold nanoparticle films[END_REF][START_REF] Pelton | Metalnanoparticle plasmonics[END_REF][START_REF] Girard | Near fields in nanostructures[END_REF][START_REF] Tripathy | Acousto-plasmonic and surfaceenhanced raman scattering properties of coupled gold nanospheres/nanodisk trimers[END_REF]. We expect similar behavior for the ribbons, and we have considered ribbons of length 45 nanoparticles, of the order of the length of the measured needles 45 × 4.8nm = 216nm. AFM data show widths of the order of 30 -40nm (Figure 3c). Taking into account the convolution by the AFM tip of curvature radius around 10nm [START_REF] Rodriguez | Probing the probe: Afm tip-profiling via nanotemplates to determine hamaker constants from phaseâ"distance curves[END_REF], the exact width values can not be obtained, only evidencing widths equal to or smaller than 5 nanoparticles. We note that the red-shift of ribbons saturates more slowly than chains and that the red-shift of a ribbon with a length of 45 nanoparticles is not completely saturated but very close to saturation. Ribbons are associated with a LSPR wavelength for polarization parallel to the ribbons larger than the one for polarization perpendicular to the ribbons (Figure 5). For ribbons of disordered orientation, the average LSPR is proportional to the sum of both extinctions and is shown in Figure 5 for a width equal to 3 nanoparticles (s = 0.56nm). The spacings between nanoparticles accounting for an average plasmon band at 565nm, are summarized in table 1 for ribbons widths ranging from 1 to 5 nanoparticles. We can already conclude that s is smaller than 0.6nm, which suggests a strong interdigitation/compression of the alkylthiols between the nanoparticles. The anisotropy of LSPR bands observed in the presence of cholesteric films is associated with ribbons that, on average, are oriented perpendicular to the anchoring direction. However, they are not strictly oriented perpendicular to the anchoring direction since a larger anisotropy than the maximum 18nm would be expected between the polarization parallel to the anchoring direction and the perpendicular polarization (Figure 5). If we consider the spectra of figure 2a, we can interpret the observed LSPR anisotropy of 18nm, as a consequence of some disorientation of the ribbons, using the property that the final extinction can be approximated by a linear combination of extinction parallel and perpendicular to the ribbons long axis. We will now explain briefly the theoretical argument underlying this property.

The extinction efficiency (cross-section normalized by the geometrical section) for an arbitrary incident polarization can be written as [START_REF] Bohren | Absorption and scattering of light by small particles[END_REF]:

Q(θ) = Q par cos 2 θ + Q perp sin 2 θ + Q par,perp sin θ cos θ, (1)
where θ is the angle between the ribbon's long axis and the incident polarization vector. Q par and Q perp are the extinction efficiencies for incident polarization parallel or perpendicular to the ribbon's axis respectively. The term Q par,perp , is the contribution due to cross-polarization (offdiagonal terms in the scattering amplitude matrix are nonzero in general). We consider now the needles to be disoriented with respect to the incident polarization vector. If n(θ) is the distribution function of the angles, we can write the average extinction efficiency as

Q = π 0 n(θ)(Q par cos 2 θ+Q perp sin 2 θ+Q par,perp sin θ cos θ)dθ.
(2) For a uniform probability density function of the type n(θ) = 1/π (our hybrid films after evaporation), we get:

Q = (Q par + Q perp )/2. ( 3
)
In the case of a non-uniform distribution, the cross-term does not vanish in general. However, for the ribbons considered here, we found that the factor Q par,perp is negligible. This simplifies the analysis greatly, since now we can write the very simple equation:

Q = Q par C + Q perp (1 -C), (4)
where we used the fact that the distribution n(θ) is normalized to one. Hence C = π 0 n(θ) cos 2 θdθ, Q par and Q perp being calculated. We can now use this equation to fit the experimental data and estimate the alignment of the ribbons (C = 0.5 being completely random and C = 0.0 or C = 1.0 being perfectly aligned).

We have considered 5 cases to account for the extinction spectra of figure 2a, each case being associated with a given ribbon width, ranging from 1 to 5 nanoparticles. The results are summarized in table 1. These results give a measure of the disorientation of the ribbons along the CLC stripes, around C = 0.65. They suggest that the areas where anisotropy is maximum (figure 2a) could display homogeneous ribbons width and inter-particle spacings. We can not establish their exact values. However, from Table 1 it appears that the spacing obtained for the initial cholesteric film (before evaporation) is very similar to the one after evaporation. Only a small spacing reduction after evaporation is evidenced, which on the other hand depends on the considered ribbon width. For chains or thin ribbons with 2 nanoparticles-width, a very small reduction is associated with the LSPR data. Furthermore, because we expect a mainly constant inter-particle spacing in the initial cholesteric film, the position dependent anisotropy can be interpreted in two ways: 1. for a constant value of the aggregate width, the disorder varies (varying C from place to place), or 2. for a nearly constant C value, the width varies together with a possibly varying local nanoparticle concentration. More precisely: to maintain the same anisotropy (for example the ∆λ = 18nm value of figure 2a), C has to increase (better alignment) together with the ribbon width. This is expected since chains have a higher aspect ratio than wide ribbons and the former are thus intrinsically more anisotropic. This is why we can expect a majority of thin ribbons in the area of maximum anisotropy (∆λ = 18nm) with C values around 0.64. With the hypothesis of mainly constant C value, the observed average smaller anisotropy ( ∆λ = 10nm) could be the consequence of an increasing number of wider ribbons when local nanoparticle concentrations increase.

Needles as the result of trapping in specific areas

In order to understand the origin of the needles, the first question concerns their localization at the CLC/air interface or within CLC bulk. We have demonstrated by AFM that, even after evaporation of the majority of the liquid crystal film, most of the needles are still covered by CLC molecules. This suggests that they are composed of nanoparticles which have penetrated into the CLC film, in contrast with segregation at the surface. This latter process occurs for micrometrical colloids at cholesteric/oil interface [START_REF] Pawsey | Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal[END_REF][START_REF] Lintuvuori | Colloidal templating at a cholesteric-oil interface: Assembly guided by an array of disclination lines[END_REF]. For nanoparticles it occurs in cholesterics after long annealing [START_REF] Bitar | Cholesteric liquid crystal self-organization of gold nanoparticles[END_REF] or for nanoparticles originally dispersed in bad solvants for liquid crystals, like water [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF].

The second question directly concerns the origin of the needles. It is remarkable that the two kinds of nanoparticles, nanoparticles of diameter 20nm covered by PVP polymer, studied by Ayeb et al. [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF] and the present nanoparticles of diameter 4.2nm with grafted dodecanethiol, both lead to formation of elongated aggregates of limited size, oriented on average by the CLC matrix. This is consistent with a minimum of energy associated with a trapping of nanoparticles in a favorable area. One can not exclude that kinetic issues also are involved and that elongated aggregates only correspond to metastable states. On the other hand, our results reveal a more anisometric shape for the aggregates formed from small nanoparticles. In order to understand this difference, let us recall first that in nematic 5CB in the absence of any localized deformation of the liquid crystal director, isotropic aggregates, possibly large, are formed from small nanoparticles coated with dodecanethiols [START_REF] Qi | Miscibility and alignment effects of mixed monolayer cyanobiphenyl liquid-crystal-capped gold nanoparticles in nematic cyanobiphenyl liquid crystal hosts[END_REF], in contrast with large nanoparticles, of diameter larger than 30nm [?]. This shows a strong tendency for nanoparticles in the nematic film to aggregate, at least partially under the influence of Van der Waals forces. Comparison between nanoparticles size (4.2nm) and helix pitch (p = 5.5µm) shows that, in the CLC, in the absence of any specific distorsions, the environment around the nanoparticles is locally a very weakly twisted nematic. We thus would expect formation of isotropic aggregates like in 5CB nematic films. However the observation of anisometric aggregates of limited size and on average oriented by the CLC matrix suggests that aggregation is balanced by a phenomenon of nanoparticle trapping. For large nanoparticles, cluster formation would occur prior to trapping, leading to formation of only slightly anisometric clusters. For small nanoparticles, we observe mainly long needles (with lengths of several hundreds of nanometers) with small width (not larger than 5 nanoparticles). Such an anisotropy is not compatible with bulk aggregation. This suggests that trapping would preferentially occur in a specific anisotropic area prior to Van der Waals induced aggregation. Aggregation would occur within this anisotropic trapping area in a second step, the complete process being associated with solvant evaporation. It is known that distorted liquid crystal areas attract nanoparticles due to the reduction of elastic energy accompanying the replacement of a distorted volume by a nanoparticle. Nanoparticles are thus attracted by distortion gradients [START_REF] Voloschenko | Effect of director distortions on morphologies of phase separation in liquid crystals[END_REF][START_REF] Senyuk | Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals[END_REF]. In CLC it has been shown that even polymers can be attracted by distorted areas [?]. For nanoparticles this is expected to occur indepedently of the nature of the nanoparticles, but the attraction efficiency has been shown to depend on the anchoring around the nanoparticles [START_REF] Rožič | Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation[END_REF][START_REF] Senyuk | Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals[END_REF]. This may at least partially explain the better trapping observed for the small nanoparticles in CLC since they were covered by alkylthiol, in contrast with the large nanoparticles covered by PVP polymer [START_REF] Ayeb | Gold nanoparticles in a cholesteric liquid crystal matrix: selforganization and localized surface plasmon properties[END_REF], the nanoparticle size also coming into play.

Trapping sites for the needles

It is now well-known that, once nanoparticles are attracted, stable sites of trapping can be at liquid crystal topological defects. This is due to the particularly large energetic advantage associated with the replacement of a defect core by a nanoparticle. Stable sites have been evidenced for topological defects localized on micrometersize colloids in nematic films [START_REF] Senyuk | Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals[END_REF], for disclinations in blue phases [START_REF] Yoshida | Nanoparticle-stabilized cholesteric blue phases[END_REF][START_REF] Karatairi | Nanoparticle-induced widening of the temperature range of liquid-crystalline blue phases[END_REF][START_REF] Rožič | Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation[END_REF], screw disclinations of twist-grainboundary phases [START_REF] Cordoyiannis | Different modulated structures of topological defects stabilized by adaptive targeting nanoparticles[END_REF] as well as for smectic dislocations [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF][START_REF] Milette | Reversible long-range patterning of gold nanoparticles by smectic liquid crystals[END_REF]. To better understand the nature of the trapping sites in the present CLC, it may be useful to compare trapping within the CLC film and trapping within smectic dislocations for the same nanoparticles.

We thus have trapped our 4.2nm nanoparticles in linear patterns of smectic dislocations of 8CB [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF], expected to be associated with a linear dislocation core of diameter several nanometers and composed of disordered molecules [START_REF] Klman | Points, lines and walls in liquid crystals, magnetic systems and various ordered media[END_REF]. We have obtained chain-like aggregates along the dislocation patterns associated with a LSPR band at 553nm for light polarized parallel to the dislocations, whereas the LSPR for light polarized perpendicular to the dislocations remains close to that of isolated nanoparticles (516nm). Analysis with Generalized Mie theory gives an average distance of 0.98nm between nanoparticles, larger by more than 50 percent than the distance obtained in the CLC. Moreover, when a simple monolayer of nanoparticles is formed in the absence of liquid crystal by depositing a drop of nanoparticles in toluene on the rubbed PVA sub-strate, we observe a LSPR band of 556nm, independent of the light polarization. This band does not vary with the nanoparticles concentration. It consequently does not vary with the size of the domains of the monolayer. This implies that the monolayer is formed of 2D domains of coupled nanoparticles that can be considered as infinite for a simulation of the monolayer LSPR since their physical size can be considered as beyond the coupling saturation for LSPR. As shown by dipole approximation calculations performed for an infinite hexagonal network of nanoparticles in a medium with an optical index equal to that of alkylthiol, the inter-particle spacing is s = 1nm. This suggests that, in smectic defects, the spacing between nanoparticles is very similar to that in the monolayer network and may be mainly mediated by the same attractive Van der Waals interactions between nanoparticles, as well as steric repulsive interactions. This is not the case in CLC.

In CLC, compression of the nanoparticles spacing imposes a larger interdigitation between the thiols around the nanoparticles. This compression must be imposed by the surrounding CLC and confirms localization of the needles within the CLC film. This compression may arise from the disorder induced by the nanoparticles in the cholesteric sample. If the trapping area is not fully disordered, the presence of nanoparticles locally induces disorder in the liquid crystal, entailing an additional energetic cost that is absent in topological defects with large enough disordered cores. Consequently in CLC, the distance between two nanoparticles must be as close as possible to reduce the disordered volume around the nanoparticles in contrast with smectic disordered cores [START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF].

A simple calculation can be made to account for nanoparticles that are further attracted by the 0.4nm difference that is observed between nanoparticles spacing in nanoparticle monolayers without liquid crystal or in smectic defects vs. in CLC. If we calculate the Van der Waals interactions between two 4.2nm gold spheres in a thiol environment, for a spacing 0.98nm, an energy of 1.5kT is obtained. For a spacing 0.6nm, an energy of 4.5kT is obtained [START_REF] Khan | Self-assembly of ligated gold nanoparticles: Phenomenological modeling and computer simulations[END_REF]. In the monolayer, Van der Waals attractions are balanced by steric interactions that do not allow for a reduced spacing. Due to the alkanethiol coating, allowing quasi-no LC to penetrate between the nanoparticles, we can consider that the Hamaker constant for the nanoparticle monolayer on PVA is similar to that for the nanoparticle needles in CLC, as only the host medium has changed. The CLC compression must be significantly larger than the gain due to Van der Waals interactions in order to successfully compete with steric repulsion associated with strong interdigitation and compression imposed by the thiol molecules between nanoparticles [START_REF] Khan | Self-assembly of ligated gold nanoparticles: Phenomenological modeling and computer simulations[END_REF].

To obtain an increase of disorder energy of 50 times the attractive Van der Waals energy (150kT) when s decreases by 0.4nm, we need a disordered volume of E7 around the nanoparticle equal to approximately 50nm 3 . This volume is calculated using the heat capacity of 8CB [START_REF] Thoen | Temperature dependence of the enthalpy and the heat capacity of the liquid-crystal octylcyanobiphenyl (8cb)[END_REF], which has been shown to be very close to 5CB, the major component of E7 [START_REF] Iannacchione | Specific heat dependence on orientational order at cylindrically confined liquid crystal phase transitions[END_REF]. This disordered volume (50nm 3 ) corresponds to a shell of disordered liquid crystal of the order of 3.5nm surrounding the nanoparticles. This liquid crystal disorder induced in the presence of the alkylthiols around nanoparticles has previously been suggested to explain the observed attraction of these nanoparticles by distorted liquid crystal areas [START_REF] Rožič | Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation[END_REF][START_REF] Coursault | Linear selfassembly of nanoparticles within liquid crystal defect arrays[END_REF]. It is also in agreement with the better trapping observed for the 4.2nm nanoparticles with respect to the large ones covered by PVP.

Despite the observed stability of the hybrid systems, the disorder energy may cancel the energy advantage associated with trapping, unlike the case where nanoparticles are trapped in smectic dislocations. On the other hand, in the presence of a shell of disordered CLC around the nanoparticles, we expect spherical aggregations of the nanoparticles to be of smaller disorder energy than ribbons. Spherical aggregation occurs in nematic liquid crystals [START_REF] Qi | Miscibility and alignment effects of mixed monolayer cyanobiphenyl liquid-crystal-capped gold nanoparticles in nematic cyanobiphenyl liquid crystal hosts[END_REF] but not in our distorted CLC. The large difference between the disorder energies of nanoparticles aggregated in spheres and nanoparticles aggregated in ribbons can be compensated by the advantage associated with the localization of the nanoparticles in topological defect cores. It can not be compensated by a localization of nanoparticles only in elastically distorted areas. Two interpretations are possible to explain such a result. First nanoparticles are trapped by topological cores. These cores must be significantly smaller than the nanoparticle size. Thus molecular disorder is induced around the nanoparticles and compression of the nanoparticles spacing occurs. Second, needles may not be trapped by disordered defect cores but by highly distorted cholesteric areas. This would correspond to a metastable state. For example, at a distance D from the center of curvature of a curved nematic, the replacement of the distorted liquid crystal constitutes an elastic advantage that can be roughly estimated to be 4KR 3 /D 2 , where K is the bend modulus, of the order of 10 -11 Jm -1 , and R the particle radius. For particles of radius 2.1nm, the elastic term is larger than kT (k is Boltzmann's constant) if D is smaller than 10nm, only two times the nanoparticle diameter. We consequently do not expect a strong stability of trapping by elastically distorted areas for small nanoparticles. The observed stability of the hybrid systems, together with the observation that no modification in the CLC texture is observed by optical microscopy more likely suggests the presence of disordered defect cores as trapping zones.

It has been shown that dislocations at the CLC/air interface, associated with the connection between planar cholesteric helix and homeotropic anchoring, can trap micrometer-sized colloids [START_REF] Pawsey | Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal[END_REF][START_REF] Lintuvuori | Colloidal templating at a cholesteric-oil interface: Assembly guided by an array of disclination lines[END_REF]. We do not believe that they are responsible for the present nanoparticles trapping since only one dislocation is expected per stripe, as shown in figure 1a. This would produce a small density of needles, not consistent with the intense well-defined LSPR usually observed, together with AFM observations of locally large numbers of needles. It thus appears natural to assume that trapping occurs in the area where the cholesteric helix rotates by 90 o to become parallel to the CLC/air interface. The trapping area may be constituted by an array of linear disclinations, on average perpendicular to anchoring direction, of width smaller than 5nm. To account for such a small width, they may correspond to the so-called τ dislocations, of singular nematic director [START_REF] Kleman | Cholesteric Liquid Crystals: Defects and Topology[END_REF]. Due to this small width, we expect formation of nanoparticle chains in a first step. The formation of chains may be followed in a second step by a localization of the nanoparticles around the chains in the elastically deformed zones around the defect cores to produce thicker ribbons (Figure 3b). This is in agreement with LSPR data which can be interpreted by a majority of thin ribbons, chains or ribbons of 2 nanoparticles width, for the more anisotropic zones (Figure 2a). The observation of an experimental extinction anisotropy of hybrid films associated with needles mainly perpendicular to the anchoring orientation for various stripe directions can be interpreted as the signature of an evolution of the 90 o helix deformation with the film thickness. The stripe orientation with respect to the anchoring direction is indeed proportionnal to the thickness [START_REF] Baudry | Arch-texture in cholesteric liquid crystals[END_REF]. This would be true at least for the range of CLC film thickness used in the present study, around twice the cholesteric pitch.

Conclusion

We show that CLC films, deformed by competing anchoring at the interfaces, planar unidirectional versus homeotropic, are characterized by the presence of linear traps for nanoparticles. They may be either highly elastically distorted areas, or oriented linear defects with disordered cores of width smaller than a few nanometers, associated with the helix rotation in the vicinity of CLC/air interface, the stability of the hybrid system being in favour of the latter assumption. These traps lead to the formation of nanoparticle needles of width smaller than 5 nanoparticles, of length larger than 50 nanoparticles and characterized by an intense and well-defined anisotropic LSPR response. The spacing between nanoparticles in the needles appears to be about 50 percent smaller than for the same nanoparticles deposited on the substrate without liquid crystal or trapped within smectic dislocation cores. This may be related to disorder induced by the nanoparticles within CLC elastically distorted areas or around the CLC defect cores. The corresponding disorder is estimated to be a disordered CLC shell around the nanoparticles of some nanometers. In the presence of topological defects, the main parameter controlling the nanoparticles spacing is the size of the defect disordered cores versus the nanoparticle diameter. These results not only gives a limit for the defect size in the CLC distorted films, but they illustrate the role of the defects nature on the crystallographic characteristics of the induced nanoparticle selfassembly. These hybrid films are finally characterized by nanoparticles strictly confined within a specific area, in the thickness of the CLC film. They open a route for a number of hybrid systems of anisotropic optical properties. They also open a path for new anisotropic electro-optical properties, with the possibility, for example, to locally heat within the CLC film trapping area, using the LSPR field excitation. This would allow for further control of the rotation of macroscopic objects under irradiation of molecular motors replacing the classical chiral dopants [START_REF] Eelkema | Nanomotor rotates microscale objects[END_REF][START_REF] Bosco | Photoinduced reorganization of motor-doped chiral liquid crystals: Bridging molecular isomerization and texture rotation[END_REF]. We finally note that studying the LSPR of trapped nanoparticles gives new information on the nature of highly deformed areas in these CLC films.

  0a CNRS, UMR 7588, Institut des NanoSciences de Paris (INSP), 4 place Jussieu, 75005 Paris, France; Tel: +33 1 44274654 ; E-mail: emmanuelle.lacaze@insp.jussieu.fr 0b UPMC Univ Paris 06, UMR 7588, Institut des NanoSciences de Paris (INSP), 4 place Jussieu, 75005 Paris, France 0c Dept. of Physics, Case Western Reserve University, Cleveland, Ohio 44106 USA 0d CNRS, Centre de Recherche Paul Pascal, 115 Avenue Schweitzer, 33600 Pessac, France 0e Université of Bordeaux, Centre de Recherche Paul Pascal, 115 Avenue Schweitzer, 33600 Pessac, France 0f Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS), Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baf, 75205 Paris Cedex 13, France

Figure 1

 1 Figure 1 (a) A typical area of the CLC sample, which exhibits the expected striped texture of a hybrid aligned CLC, (b) The director field of the aligned CLC sample. The helical axis is along the ẑ axis, perpendicular to the planar aligning surface over a thickness (D -p/2), p being the cholesteric pitch. It gives way to a rotation of the director within a thickness around p/2 from the CLC/air interface to allow the director to meet the homeotropic boundary condition at the interface[6]), (c) map of the entire sample after evaporation of the liquid crystal.

Figure 2

 2 Figure2Localized surface plasmon resonance of (a) a typical area for the CLC/nanoparticle hybrid sample (stripes perpendicular to the anchoring direction) and (b) isolated gold nanoparticles suspended in toluene. Parallel (red) and perpendicular (blue) curves correspond to the incident light polarization orientation with respect to the stripe orientation. A Gaussian fit was performed to determine the band wavelength value.

Figure 3

 3 Figure 3 (a) AFM Topographical and (b) phase data for a region with a plasmon resonance peak. (c) Corresponding topographical cross sections according to the lines drawn on (a) and (b). A high number of anisotropic aggregates are visible in the phase data below a liquid crystal layer. The dimensions of a needle (width 35nm -(bottom of (c))) are determined using the phase data (b) as this allows for precise location of the needle's edges.

Figure 4

 4 Figure 4 Extinction spectra from the selected region where AFM measurements were conducted. Parallel (red) and perpendicular (blue) curves correspond to the incident light polarization orientation with respect to anchoring orientation.

Figure 5

 5 Figure5Extinction spectra calculated in generalized Mie theory in a thiol index for a ribbon of length 45 nanoparticles and width 3 nanoparticles (s = 0.56nm). In blue the polarization is perpendicular to the ribbon in red it is parallel. In green the average extinction is presented.

Table 1

 1 Results of the fit using eq. 2, for different widths.

	Ribbon width	C before	before evaporation after evaporation
	( NP)	evaporation	s/nm	s/nm
	1	0.65	0.54	0.53
	2	0.64	0.57	0.55
	3	0.65	0.59	0.56
	4	0.68	0.62	0.57
	5	0.71	0.64	0.58

Acknowledgement

We would like to thank Dominique Demaille of INSP for TEM measurements and Mohammed Selmane of LCMCP (Paris-France) for making SAXS measurements. We would like to thank M.H. Li and P. Keller from Institut Curie for providing the rubbing machine. The CWRU experimental work was supported by the National Science Foundation's Condensed Matter Physics and Solid State and Materials Chemistry Programs under Grant No. DMR-1065491 and the analysis and exposition were supported by the US Dept. of Energy under grant No. DE-FG02-01ER45934. JSP's travel and expenses at UPMC were supported by the Chateaubriand fellowship and the Partner University Fund.